Vectoring and Vexeling are both pretty easy but very time consuming and tedious. I think you should not make the colors change so drastically and I would use slow transitions or gradients on something like this.

Whether you want to call it a Vector or a vexel thats up to you but there is a difference Vectors can be Enlarged with little to no resolution loss, vexels cannot, Vexels are done in photoshop while vectors are usually done in Illustrator.

The key is patience.

you can play with transparency

It's a vector.

If I work up the courage I'll start doing a real attempt in an hour or so

are you using illustrator or photoshop? are you attempting to create photo-realistic or stylized vectors? assuming youre using illustrator and working at photorealism then i would say check out gradient meshes and the blend tool as they will give you much more subtle changes in color.

yeah photoshop kinda sucks for working with vector shapes..

I think I should probably sit down and learn Illy before I jump right into vectoring, at least learn how the tools all work, that is. First time using illy :< I am a late bloomer, I know.