I haven't done that yet, as I was more worried about colorizing itself - I've made very little progress lol - the colorize function I wrote sets the hue of all pixels to the given hue parameter, then adds the given saturation and lightness parameters to the original. You could probably use similar code to only colorize certain pixels, and use it to create an AnimatedBitmap. I've just written up a variant of the core colorize function to do this: it only colorizes pixels when the pixel's position is on certain diagonal lines (which can then be incremented for each frame to make the AnimatedBitmap). So basically you could run anColorizeBitmap(frame,hue,0,0) and then colorizeDiagonal(frame,hue,0,0.4,i,3), which will make a colorized version and then make a much lighter diagonal line across the image, then add the frame to the AnimatedBitmap, on a loop. When this is stitched together it should make an animation where each frame, the diagonal line moves along, making a shining-like effect. (It's very similar to the shining effect played on the Pokémon logo in I think the GBA Pokémon games?)
I am unsure on if an AnimatedBitmap can be generated like this though. I haven't tried it, as I hadn't started implementing the actual mechanic yet, just the backend (Tera Types, how a Terastalized pokemon's type is determined, and the colorize function).
Either way, I've attached the colorize code to this post. Sorry I couldn't help more ^^"
