Is it usually true that the brighter and more colorful a fruit and vegetable is, the more nutritious it is?

Is a darker color grape more nutritious than a white grape? Same with a colored onion over a white onion? Is this usually true when it comes to nutrition? What might some exceptions be?

yes!
"As an easy rule, the darker and brighter the colour of the vegetable the more vitamins, minerals and fibre they usually contain. For example, spinach contains more nutrients than lettuce."

That is why carrots are sooo good for you.

well that is really intersting question hmm never think of it. but i think they are same but hey i might be rong. good luck and sorry.

I suppose it is true. Dark leafy green vegetables are said to be more nutritious than pale green ones. Fruits usually have more intense color (and taste) and have full nutritional benefits when ripe But I don't really know if a ripe but pale fruit is less nutritious than a colorful ripe one. Good question and I am going to research that myself.

Fruits need color in order to attract animals to eat them and expel their seeds enveloped in "nourishing" manure. Flowers use the same "tactics" as well.

Yes because dark colors imply antioxidants, beta carotene, other pigments, and not only are some of these pigments good for you, they also signify that other nutrients are present.

green = chlorophyll
purple and blue = anthocyanins
red and orange = carotenes, usually beta carotene