Moving a mask to reveal something underneath, but I want the mask to have a border around it?

So yeah, I have two different images in my timeline right now. I want a mask to move from offscreen to the center of the screen, revealing an image underneath as it does so. But I want the mask to have a border, and ideally, the border would have two colors to it. Is that possible to do in Premiere?

I have access to After Effects, if that's a better option.

Here's also a gif of what I'm trying to acheive, but again, I want a two-colored border around the mask.

Any help would be appreciated!