Updated : 2021
A quick guide to creating random fills using the random fill script in Photoshop fill command tutorial
The key thing, I have found with the random fill pattern tool in Photoshop, is that it works best when you pre-size the pattern tile according the document you wish to fill.
If you have a document that is a 1000 x 1000 and then fill with a 700 x 700 tiles then the result will not be very spectacular. If you are using a 1000 x 1000 sized document then use a pattern tile around 200 x 200 say (or 200 x 100.
Open pattern file or texture for the pattern tile for Adobe Photoshop
image menu
image size
set width to something like 200px
set height to something like 200px
OK
If the image is on a layer then you can add layer effects such as color overlay such as red overlay
Remove background layer if you can by deleting it in the layers panel
Go to the edit menu
Define pattern
Another quick note, make certain there is some transparency to get the best results as if you use just an image as the pattern tile (say filled with a background of a solid color) then you will just end up with a document of colorful pattern stamps in a square shape and at different angles etc (which is fine if that is the requirement from the random fill pattern.)
You need a selection of source tiles or stamps to use and you can quickly create your own or use the default pattern tiles supplied with Photoshop and there are many different sets available on the web
Where is the random fill pattern command in Photoshop?
edit menu
fill
select the "custom pattern" option
script option ON
script 'random fill'
Select pattern
Click OK
Go to the panel and set size etc
As with all things in Photoshop, the pattern script can be used once or multiple times. You can also combine the result with other effects such as blurs as well as adjustments, you can run the edit menu random fill and use the edit menu fade command with different blending modes etc.
The initial panel offers you the option to set the pattern to use (sadly, it is not a sequence of patterns and there are no other options to tweak the tile before the operation so make certain you select the correct tile to use as the next panel does not offer that tile again). You can set the blending mode and opacity and that is that (no scale or rotation etc option there).
Personally, I prefer to use the random fill script onto a layer. You can always just delete the layer if it doesn't work out and another feature of the layer is the transparency and how the random fill pattern only fills part of the layer and leaves the lower layers still visible and they can then be manipulated further using tools such as camera raw filter effect in Photoshop as well as adjustment layers and Photoshop styles and you can always use opacity and blending modes to blend the random filled patterns. Of course, you can run the random fill pattern command and just apply to the background layer and just layer more and more patterns and pattern stamps to that layer and that can also be interweaved with effects such as blurs to add interesting variations into the design (or re-colored between fills)
Another good thing about layers is that you can then use transparency from the layer to load a selection and then use styles etc with that or create a mask from the transparency of the layer and the random fill pattern via the layer menu and layer mask and from transparency command which can then be feathered and manipulated in many different ways to create more interesting combinations of results from the fill.
Layer
New layer
Edit menu
Fill (select "custom pattern" and script and random fill)
select the pattern
OK
Set all the settings in the next dialog such as scale etc
OK
Layer
New layer
Edit menu
Fill
and repeat over and over with perhaps different scale and density etc
Once you finally get to the random fill script panel, you will see an option for adding presets (which may or may not be useful and if all your patterns etc are the same size then perhaps it is quite useful) and you also have a selection of settings to choose from such as
set the density
minimum scale factor
maximum scale factor
max distance from path (probably disabled unless you have a path selected)
rotate pattern - randomize between 0 and 360
color randomness. The color randomization is great for changing reds into blues and greens etc at the maxim or for subtle color shifts if set low.
brightness randomness. Brightness is great for adding some randomization of light and dark artwork into the design. The combination of brightness and color is very good but the result will depend on the quality of the pictures.
You can increase the density etc and you will see more of the stamps or tiles in the preview. If the image size ratio to the pattern tile size is ok then the preview should match the actual result (as mentioned). Even if you ram the density up to 10 it may never actually entirely fill the entire preview or result with a single random fill pattern application in Photoshop so if you set it to 10 and want it to fill the entire image then run the tool a few times or just increase the size of the tiles by using the minimum and maximum scale factor settings. If you set though scale factor quite low, it will be hard for the density setting to totally fill the entire document unless you add repeat applies.
You can re-size the tiles by setting a minimum and maximum (the tiles are then sized randomly in between those) so if you have a tile of 200 x 200 then the scale factor of 0.1or 10% will result in some fairly small tiles generated (or not depending on the randomization) but if you set the maximum to 3 then you will have a lot of larger tiles and perhaps the quality will not be so great - this is something for you to decide. Changing the scaling does not affect the randomization of the positions but changing the density does (nor does the color or brightness). Sadly, there is no 'randomization' button added to the panel which can be click to change the position etc of the tiles.
The results of the random fill vary depending on the source material. You could use the number '1' as a pattern source in one apply of the fill and then '2' etc in another apply and then 3 etc to build up a very random numerical pattern / background. You can then repeat this with 3 4 5 6 7 8 9 etc or perhaps A B C D E F G H or perhaps with images of Marvel superheroes such as Thor and Spider-man and Captain America etc
You could have different images of cats and then select those in turn and fill randomly an image with different cats. Or different letters or different colored shapes etc.
It is a pity that the random fill panel doesn't allow for the change of pattern and it is a clunky two pass effect, still all number of combinations of different tile sources can be used to fill the document or selection or frame (the random fill pattern script makes for a great tool to create randomly colorful filled frame artwork)
If you create a very small object such as a 10 x 10 or less circle or square (red, green etc) and then use the minimum settings for the factors and the maximum for the color and brightness and density, you can create some interesting noise effects in Photoshop using the random fill pattern script.
May require a few repeat applies to fill the entire image or selection.
Always slightly annoying is that the fill has no repeat feature but a very quick workaround is just to record it as an action and then re-apply over and over that way to speed things up
New document (10 x 10)
Ellipse tool (or any other small design such as 3D shapes etc)
Set color to red
Set option to shape layer
Create small shape
Delete background via layers panel
edit menu
define pattern
go to a new document such as 1000 x 1000
edit menu
fill (custom pattern, script ON and 'random fill')
select the small pattern
OK
Set the density to high, scale to 1 etc
Set brightness and saturation settings as required
OK
You can also then apply blurs and other effects such as oil paint or camera raw etc
You now a have a quick noise pattern. Might take some time to process if there are 1000s of pattern tiles to generate
Ok, actually quite useful as you can run the script with a path
Simply create a path using the pen tool and make certain the path is selected Via the path panel) and then run the edit menu and fill etc and instead of filling the entire document or selection, the pattern follows the line of the path and and as the path can be any kind of design such as a spiral or zizgag artwork etc, you can create some really interesting artworks just by using that feature. If there is no path, the path option in the panel will be disabled
Go to the custom shapes tool in the toolbar
Select a custom shape
Go to the top bar and select the path option for the shape
Draw the path
Select the path in the path panel
Go to the edit menu
Fill
Select the custom pattern option
Set script ON and set the script to 'random fill'
Click OK
Set the scale / density etc
Set the maximum distance from path (now enabled)
Click OK
May require a few re-runs of the tool to generate enough pattern tiles from the path / random fill
The random fill script is a script, in javascript, and can be found on your machine and edited and tweaked in many ways depending on your skill in Javascript.