Best to keep illustrations super simple as the effects considerably slowed down my computer. Ideally I'd like the grain pattern to be a bit bigger.
I tried it with colour. One thing I found out is if you modify the grain intensity and contrast (in the settings), it alters the output colour. So sometimes I'd have two patches with the same colour grain but they would show up differently! Took me a while to figure out why! So this first attempt at colour was a bit too busy - even though I had the palette limited to 3-4 colours.
Scaled down the composition, switched the colour scheme and edited the grain pattern to be "sprinkles." I could spend a bit more time modifying the pattern location and scale. Thanks for the feedback!