As a native french speaker, I can assure you that both sentences are correct, although “C’est du piment rouge!” sounds more natural to me.

“C’est un piment rouge” implies that you are speaking about a whole red pepper whereas “C’est du piment rouge” implies a more vague amount of pepper.

It’s the equivalent of “It’s red pepper” vs “It’s a red pepper”.

Sur l’étal d’un marché de plein vent, c’est un piment rouge.

Au rayon des épices du supermarché, c’est du piment rouge.