Yep, there is no quick fix on reds, you can't cross out to other colors to improve it. You need to find one good rabbit with the color you want and raise out all the babies you can to a senior coat. Watch both top color and carry through for intensity. Beware smut, not sure what breed you're working on but you have to watch out for fade on the skirt and lap spots on some breeds (like mine).
Red can be a color that develops its intensity slowly in my experience, so don't ditch those babies until you see their senior coat coming in!