Editing illustrations once they are completed is a rare occurrence, but I have just finished working on a brief where the client then wanted to edit all the colours in the finished artwork, which is thankfully rare and a little odd too.
Editing illustrations: As you can imagine this can involve a great deal of extra work, which you are not being paid for really. This is especially true if you work conventionally. However this is where Photoshop can come to the rescue to all conventional artists. This of course is based in the assumption that you supply a client with digital versions of your work.
In Photoshop with a bit of effort there are several different ways you can isolate an area of artwork and then alter the colours completely. This can be done by either using the magic wand on a low percentage setting or using the lasso. You can then use colour balance options, photo filter, hue, etc to then alter the colour completely. Thus saving you from having to redraw the illustration from scratch. This is also useful when they want the changes within 24 hours or less, which happens.
Anyway to show you what I mean, here is a piece of artwork that has not only had it's colour range changed, but the size of the bed the child is sleeping on. All achieved digitally, using Photoshop.