The tldr; on this is that I got some gel medium to mix the thermochromic pigment powder into and I like the way it's handling a lot better than mixing it into white paint.

Now then.

In the last set of tests, I had mixed the thermochromic powder with white paint and that worked okay, but meant that colors like black and dark blue weren't as dark as they could be. So I got some gel medium (Golden Soft Gel Matte), which is a translucent, colorless medium that can be used to make regular paint more transparent, or can be used the way I'm using it - to mix pigment powder into to make paint.

The results are much more to my liking.

You can see in the above picture that although the gel medium is still visible when the thermochromic pigment is heated and colorless, it's a lot less opaque than the white paint version.

Also of note is that the thermochromic pigment goes *mostly* colorless. It's not completely gone, especially for the deeper hues. 

The gel-based thermochromics are the horizontal lines in the above gif, and the lines are painted over and then under the regular paint vertical lines, and the animation shifts between heated and unheated. (The image only moves because I did the photos handheld - tripod next time!)

So now I need to do some experimentation with mixing the gel-based thermochromics into the regular paint and see how those transformations go when there is direct color mixing between the two.