Best color as in neon lighting in/under a red car? Or best neon hued color to compliment an already red paint job?
Either way, red is tough to match with neons... even red neons. If you're talking about lighting then I suppose white neons could fit for underbody. But if you plan on a deeper wine red for the interior upholstrey then I think that makes it even harder to make neons fit. You could go for wine/white inteior and then do white underbody but I personally think there are better combos.
If I was to add neon to a red car, I think I would go for a GT circuit look and do yellow accent paint and lighting.