Sometimes it is desirable to change the color of a reference photo by using harmonious color schemes for paintings. James Gurney has done an excellent job explaining how to achieve this by doing a Gamut Mask in this video. You should also check out his blog post on this subject, it is very informatives as well.
Here is a free interactive Gamut Mask tool made by artist Richard Robinson- check it out!
I also like his disposable palette idea - he uses a roll of freezer paper, which has a smooth surface. A genius idea!