Shape Layers. Strokes. You know how they work– add a stroke, and it'll increase in width both internally and externally from the path. But, with expressions, we can preserve the outer edge of your path and keep our stroke internal!
Here's a quick little preset that'll add a pseudo "inner stroke" to your shape layer. Now, there's no "official" way to do this, so it's a hack and a half that uses offset and some expression magic to simulate it. Depending on how tight your shape corners are this can break things a little, but otherwise it's pretty solid.
Apply the preset to your shape layer, and it'll add in an offset and a new stroke. The only things you need to worry about here are
stroke width and
stroke colour. Duplicate "Inner Stroke - Stroke #X" to add more inner strokes to the stack, however the more strokes you get the more likely it is to produce strange results.
Download and play!