I think red. It was red on my '52, but I changed it to blue. I used blue glass that I carefully cut to fit the space that the original crumbling plastic lens fit into. I elected to go with blue to match the common modern color for high beams. Red just seems like a warning light color to me.
Here is a photo I took of my cluster after restoration. I used a flashlight to illuminate the high beam indicator.
If you go with glass, you can find the color you want in the stained glass section of a big box craft store or similar.
