I have uploaded a new version that has the centerline and also press V (for "Same Values") to pick same colored bits (very neat).
I wish there was a way to overcome graphics card's limitations and always have the best quality for the parts.... Well, when the part kit is finalised I'll try making a smaller subset for smaller parts. Also the way C2 slaps them together on export isn't that great, I wish it just took my spritesheet and used that wholesale.
Undo is quite hard... I'll probably figure it out, but for now... save often