Way back when I did an ACEO series with fantasy creatured in rainbow colours. This set turned out better, particularly since I allowed myself to use other colours for the background. Yellow is particularly tricky for me, because I tend to sort things as "yellowish orange" or "yellowish green" pretty quickly.
Those are all ACEOs - meaning 3.5 by 2.5 inches - on 600+ g/sqm bristol board, made using watercolours, colour pencils, matte acrylic paint, more colour pencil, acrylic ink, and a bit of PITT artist pen.
They're available for sale, for 8$ each or if the first asker wants all, 40$ as a set, shipping included. (Or they'll go up on Etsy in a while, where shipping will be extra)
I'm also open to trading, though. :)
Comments and feedback are always appreciated!