See layers beneath the solid I'm editing in After Effects I have imported a photo into one layer and I created an opaque solid on top of it.
I want to animate the solid with the Brush tool (Write on).
When I enter layer edit mode for the solid I can't see the photo layer underneath it, and hence can't use the photo as a guide for the Brush tool.

Is there a way to increase layer transparency so I can temporarily see a guiding layer beneath it?
Don't apply the brush tool to the solid - apply it to your photo instead.  There is an option in the tool to choose whether to apply the brush over the image, or to reveal the image.  When you're drawing your brush strokes, set it to apply over the image.  When you're ready to animate the strokes, set it to reveal the image.