AKVIS Coloriage allows colorizing B&W photos and replacing colors in color photos. Just indicate the desired colors by the stroke of the brush; the program does the rest of the work: recognizes the object's border and tailors the new color to the grayscale tones of the initial picture.
Coloriage v.11.0 Mac (dmg) - Standalone App (~58 822 Kb)
Coloriage v.11.0 Mac (dmg) - Plugin (~58 822 Kb)
Userguide (pdf) (~4 959 Kb)