To change the MIDI notes appearance, there is only one file involved :
midi_note_colormap.png. This one should be in the subfolder containing all the image files of the theme you want to edit.
The problem is that its structure is a little complicated. For more details about it, I suggest to see what is in there, p.51-52 :
http://www.houseofwhitetie.com/reape...mers_guide.pdf.
Remember that the 64 vertical pixels used for each selected/unselected notes will be stretched accordingly to the user actions and, unless I have missed something until now, we cannot use pink lines on this file.
HTWH...