Trying to paint outside of mask

If you want the new paint strokes to appear on top of the layer with the mask, just open the layer in the Layer Panel (The Paint workspace makes this easy) and change the Paint/Channels setting from RGB to RGBA.

If you want to maintain the clean edges of the mask and paint behind it follow this workflow:

1. Duplicate the layer you want to paint on; name it Paint On Me or something like that
2. Move the duplicate below the original layer
3. Open the Paint workspace and then select the Paint On Me layer, and paint away using the RGBA Channel Setting in the Paint Panel

That's all there is to it.