To answer your title question: Same meaning but grammatically wrong. You may omit 色 when used as an adjective. The blue sky = 蓝天 or 蓝色的天. Here in vernacular rules, you must add 色 and 的. To be more technical, 蓝 here becomes adjective for 色 and taken together, becomes a noun phrase to describe the noun sky. So the exact translation would be "blue is the color of the sky". In answering your second question, 色 may be omitted in literary or poetic situations: "蓝天白雲海水青 The sky is blue, the clouds are white and the ocean's waters green"