I spent an hour just to make the partial colour and imagine check it didn't have imperfections nor colour outside Celestia just to edit the rest and tell the colours between Celly and the ambient.
I certainly did it but I mostly have failed trying to put her on white or something of good looking.
I show the partial colour,the middle one is the closest I got to decency and finally,I ended up making the easy way. After editing, I couldn't find a way to restore the brightness and it only ended edgy(red lines) most of the time.
Unsatisfied with this one but it represents how monochrome images can get more lively. They take more time though.