色彩 = color or facial colors indicating the moods as in "a darkened face" in English. 颜色 = tone and hues of the color. In XiaoDeng's examples, 这幅画色彩太浓了。This painting is too colorful or The hues and tones of this painting is too much. (一幅画中)鼻子这块的色彩太淡了 The tone of the nose in this painting is too pastel. 大海深处其实也是色彩缤纷的哟 Actually, there are many hues in the ocean! So substituting "color" for tones, hues in the sentences is understood by not as preferably good.

That's a hard one. I'll just try to provide some points. 颜色is a very common word, just like color in English, a very neutral word. 色彩 is more literary. More often than not, it is frequently used related to certain emotions, negative or positive. 这幅画色彩太浓了。(negative) (一幅画中)鼻子这块的色彩太淡了。(negative) 大海深处其实也是色彩缤纷的哟！(positive) You can substitute 颜色for 色彩 in the above examples, but 色彩would be better. Of course we can use the two interchangeably in almost all cases. (I can't think of any exceptions for now.) It's just one is more suitable for everyday life, the other more common for writing. They're actually the same thing.