If you've dyed your hair red and want it medium brown, can you cover it with a semipermanent?

I think you can minimize the redness, at least. If you want it a more neutral color, I believe you can tone down the red with an ash-toned shade.

I'd start with the lightest shade that's close to your current color (like light ash brown), and if it doesn't cover, try one shade darker (wait a few days).

You may want to find a good colorist, though.

If you've dyed your hair red and want it medium brown, can you cover it with a semipermanent?

only if you keep it at the same level

semipermanent hair color will not lighten hair

so your answer is yes