For whatever reason, plagiarism is major taboo in art circles. Getting caught lifting something verbatim can ruin your reputation overnight, so it pays to be safe and always give credit where credit is due, and even give credit when your not sure it's due. All it takes is a single sentence or two to site your sources, so you might as well.
On a related note, companies license colors from Pantone for their logos and sue other companies who use similar colors, whether they intentionally stole the color or not. Pantone basically owns a big-ass color palette and enforces it through trademark litigation.