In Japanese, the four colors 赤, 青, 黒, 白 are special. They are nouns, and also the stems of i-adjectives. The other colors don't have conjugation forms, but 黄色 and 茶色 are the only two exceptions. These colors can work as i-adjectives when used with 色.
○赤い 青い 黒い 白い
○黄色い 茶色い
×黄い 茶い 緑い 紫い 桃い
×緑色い 紫色い 桃色い
FYI, 赤[aka], 青[ao], 黒[kuro], 白[shiro] have the same etymology as 明るい[akarui: bright], 淡い[awai: pale], 暗い[kurai: dark], 顕し[shiroshi: clear/sharp], respectively.