How can I make the following transition in After Effects (or another software maybe)? I have the different colours of the piano as pictures, I just don't know how to make this transition! Please check the video attached. Thanks!
I made a little video, check the link.
An easy web solution is using a svg with fill transition and css, simpler than create a gif.
You can use http://ezgif.com/maker
Just select the "crossfade frames" option and it will generate the intermediate frames for you automatically. You can change the duration and frame count according to your needs. I threw together a quick example from 2 frames I took from GIF in the other post you made. If you upload the originals in png format, it should have better quality.