You can get Mark Christiansen's book "After Effects CS4: Visual Effects and Compositing" you'll have all you need in it to do exactly what you want.
What you can do is duplicate your layer, roto your walking man on the top layer, and add on the bottom layer a Color Correction > Tint effect and animate by hand the effect transparency setting, or a Color Correction > Color Balance (HSL) and play with the saturation setting.
Thanks for answering my question. I ordered Marks Book and have a question on this. Can I do this without green screening the man walking and adding him to the upper layer? Like can I dupicate the first layer that has the man walking through downtown, say, and roto him in a layer above, use the rectangle tool and cut the man out on the above layer and make him transparent. I don't where I'm going with this, but can you kind of tell me how or point me to a tutorial on doing this without any green screen.