Select the paint brush tool in the tools panel of Affinity Photo and then go to the panel to select a design to use, I would suggest one of the ink ones as they are pretty good and you can then double click the entry and change the dynamics section and the hue jitter, that is all. Set the color to red for the stroke or green or blue etc and then apply and apply it multiple times across the image and avoid going over the edge of the image as you don't want to create a sharp break in the art (unless you want that).
Once you have finished, you have a pixel layer, this layer can now be turned into a pattern via the layer menu and create pattern layer from selection and once you have that, delete or hide the original layer. You can now resize and rotate the artwork but also you can hold down the alt / option key to duplicate the artwork layer and repeat that multiple times and place it across the image in different locations as well as re-size and rotate and shear it to create a variety of different designs (all hopefully very colorful across the image).
I would suggest saving your work to an afphoto format as I have found the merge selection feature occasionally crashes the application so it is always best to be safe. Once you want to merge them, go to the layer menu and use the merge selection so you can merge all the current selected layers. You now have a single design to work from.
Go to the top left (move tool selected) and set the mirror option to on and then go to the filter menu and distort and mirror and use that with a setting of 12 or 16 and also change the origin point as well as change the input and output settings, literally millions of designs can be created (it is a pity that there is no auto save feature to save the designs as you move the cursor and settings or an auto-movie capture feature). Once you are happy with your design, remember to apply it and save your work.