Hi there! 
I am new here, i am portuguese, i am 22 years old, and to finish my multimedia engeneering course i have choose a computer animation.
That computer animation is a join of a scenario made in sketchUp&Cinema4D and the animatio made in Reallusion iClone.
Well, to join the two different parts i have choosen AE software (actually is After Effects CC) and i have two questions that i cant find the answer:
1º: Well, as i said the animation and scenario are in a different project, and i have joined both in AE. But the scenario is not always just a landscape. Sometimes is a building that the actor walks on it, so i have (in some way) to put the actor "in" the building, and if i was working with images i would easly solve that problem: select and extract to another layer what i want to be in first place, or to be the landscape. At the same logic, how can i do this in After Effects CC?Can i extract some objects by color as i can do in Photoshop?
2º For storytelling reasons, i would like that , in some moment, everything turns gray , and the actor is colored. but there's more! i would like that the actor leaves a colored path (the path that he is taking in animation). Is there any way that i can do it?
If i cant do it, i have to re-build the entire animation in Cinema4D.
Thank you very much!
Peace 