How do you spell yellow in french if its feminine?

It's important to note that while many colors in French have gendered forms (like bleu/bleue), jaune is one of the few that doesn't change depending on the gender of the noun it modifies.

So, you would say "une robe jaune" (a yellow dress) or "une voiture jaune" (a yellow car), regardless of the fact that "robe" and "voiture" are feminine nouns.