How to assign roto to another layer in After Effects I'm rather new to AE but I have to do some complicated things for school since we came up with such a hard concept.

I have rotoscoped a part of the main footage, now how can I apply this roto mask (at least that's how I understand roto's function) to a solid layer?

My solid layer is something I painted on and I want it to stay within the bounds of the rotoscoped object.

Any help on this would be great!

Thanks.
Put a layer underneath it and set the lower layer's "TrakMat" to "Alpha Matte".  If you don't see the "TrkMat" dropdown menu to the right of the source name, click the "Toggle Switches/Modes" button at the bottom of the screen.