user6519: 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?

user4426: 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.

user6476: The selected answer already addresses your question with this specific effect, but if you run into other similar situations where you want to see your results while editing, you can either add another Comp Viewer window, or lock the one you're using. 
At the top-left of the comp preview, there's a small dropdown where you can choose which comp to display, or open a new Comp Viewer.

Next to this dropdown is a small padlock icon. When unlocked, the Comp Viewer will switch to show whichever comp timeline is currently active. 
When locked, the viewer will keep showing whichever comp you selected/locked, so you can change comp timelines, do whatever editing you want in the selected comp (effects, text, animation, etc), and view the results in the parent comp immediately. (Note that if you click to directly edit an image, solid, etc it opens a new "Layer Viewer" with the same padlock, so you could edit that layer in the same way while viewing the comp separately.  


6519: <p>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.</p>

<p>Is there a way to increase layer transparency so I can temporarily see a guiding layer beneath it?</p>
 4426: <p>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.</p>
 6476: <p>The selected answer already addresses your question with this specific effect, but if you run into other similar situations where you want to see your results while editing, you can either add another Comp Viewer window, or lock the one you're using. </p>

<p>At the top-left of the comp preview, there's a small dropdown where you can choose which comp to display, or open a new Comp Viewer.</p>

<p><img src="https://i.stack.imgur.com/aX0eP.png" alt="New Comp Viewer in AE"></p>

<p>Next to this dropdown is a small padlock icon. When unlocked, the Comp Viewer will switch to show whichever comp timeline is currently active. </p>

<p>When locked, the viewer will keep showing whichever comp you selected/locked, so you can change comp timelines, do whatever editing you want in the selected comp (effects, text, animation, etc), and view the results in the parent comp immediately. (Note that if you click to directly edit an image, solid, etc it opens a new "Layer Viewer" with the same padlock, so you could edit that layer in the same way while viewing the comp separately.  </p>

<p><img src="https://i.stack.imgur.com/ZqeFM.png" alt="enter image description here"></p>
