Affinity Photo : CMYK Procedural Texture Filter Tutorial
Instead of color format of RGB, you can set the color format to CMYK when you create a new document (or using an existing CMYK document).
Once you have your new document or existing, you need a layer and it needs to be filled with something so you can do that via the layer menu and new layer and also edit menu and fill with white or green etc.
Go to the filter menu and color and procedural texture and you will see that the dialog is the same as in RGB but if you go to the presets section there are no presets added by Affinity (which is plain odd). I tried to copy some over from RGB and the ones they have failed to work for some reason though some of the functionality does work.
Go to the + on the left and add an equation and the first one will bet set with C (cyan) and then click again three times to create ones for the magenta etc. Once you have them all, you can enter an equation such as rx, relative x, it does little but it does show the filter works.
You can also enter oscsin(rx * a) where 'a' is a custom input. I would suggest going to the R field at the bottom of the dialog and clicking that four times to generate a, b, c, and d and you can set those to 5, 6, 2, 3 etc You can also now change the equation and set it to oscsin (rx*a*0.01) and the reason for the 0.01 is to reduce the a field to a lower value instead of entering decimals in the custom inputs, repeat this with the same entries in the other channels so you have oscsin(b*rx*0.01) etc. You can now modify the a and b etc settings and generate different color lines.
You can also go to the presets menu in the top right and create a new preset so you can save this for future use, you now have your first preset for the filter.