Hi!
This video is a pretty clear explanation of how to set up a timeline for animation in photoshop:
I export my line animation from AP as a PNG still sequence, start a project in Photoshop with a timeline (set to the same frame rate as my animation paper export) then just import the AP PNGs into that PS project. I then start adding new video layers underneath the line animation and use different brushes and colors to fill in color under the line animation frame by frame. For areas with holds, you can create a new regular layer in the PS project and color once then stretch that layer in the timeline to match the line drawing hold. One other way I’ve done simple stuff is to color (with the existing red green blue colors and a fat brush) directly in AP, then export those color layers separately- you can then import those into after effects and “change color” to the layer to make it any color you want.
It’s all clunky for sure, which is why I can’t wait for AP to offer coloring!!!