Rainbow Negative Space Tree

So I learned this technique way back in high school, and I often find myself doodling using it, so I thought I'd try something new.
It looks like I've found something I can draw more than reasonably well. Dead/deciduous trees. I've decided to do a series of these, each with a different colour for the negative space, and each with a different sort of tree. It's going to be an interesting journey. Excuse the crappy photography, that is not a strong point for me.
Here is the work-in-progress: Here