So far, experimenting with limited colour palettes, squiggles that don't touch each other, some layers...  

 

 

Paint roller and fountain pen

 

 Overlapping

 

 The rules for this one was supposed to be no different colours touching, but I forgot at one point.

 

No touching

 

Paint roller and fountain pen

 

Unbroken line

 

 

 

Eyeballs