I've been playing around with different ways to color images. The photos here aren't an exhaustive list for media with which to color, but it does give you some side-by-side comparisons that I found interesting. I also did the same technique after stamping the image in both black and brown on some of these just to see how different they would look. I wasn't at all happy with how the images using Aqua Crayons turned out, so I'm not even posting those!
Each media has its purpose, and not all are well-suited to the wide open areas of the cow, but it's fun to see them all (or most) on the very same image.
*This photo shows Prismacolor Pencils with baby oil and blending stumps on the left. The image on the right was colored with Tombow Markers scribbled onto a clamshell case, then picked up with a Niji Aquabrush.
*I colored the images below with Watercolor Pencils and blended with a Dove Blender. The one on the left was stamped in Noir (black) and the right one was Burnt Umber (brown).
*Left: Colored with Tombow Markers on clamshell, then picked up with Aqua Brush.
Middle: Used chalks and the little puff balls that come with the set.
Right: Used the same chalk colors, but picked up the color with a Dove Blender.
*Top: Prismacolor Pencils only--no blending other than with the colored pencils themselves.
Middle Row: Prismacolor Pencils and baby oil with blending stump, using Noir and Burnt Umber.
Bottom Row: Prismacolor Pencils and oderless mineral spirits with blending stump. Again in Noir and Burnt Umber.