Few plant colours are vitamins but as your intuition tells you it is in fact an indication of vitamin content. Most vitamins do not have colour. Riboflavin is the only one with a light greenish yellow. If you ever made cheese it explains the reason why the whey is greenish in colour. This is particularly true if you ever made ricotta cheese using the whey which basically takes out the milky whey proteins and you are left with whey water. That water is eerily greenish.
Beta-carotene the well known colour of carrots and butter (orange yellow), Broccoli has a lot of it, is technically not a vitamin. It must be digested for it to be converted to vitamin A. Lycopene, the red colour of tomatoes, is also a pre-cursor to vitamin A. This is why I mentioned phytochemicals which include non-vitamin antioxidants, plant chemicals and pigments.
So yes, plant colour and intensity is an indication of vitamin content. Other factors that assures high vitamin load is freshness, ripeness and absence of browning in fruits and veggies. Browning occurs mostly by oxidation so it is an indication antioxidants are low. Many common antioxidants are vitamins. Vitamin C is a good example. Add Vit C on a slice of apple and it won't brown.
Chlorophyll, the green colour of plants, does not convert to any vitamin but is a benefical plant pigment. Green coloured plants is usually an indication of high folic acid which is a very important vitamin that lacks in nutritionally poor foods like fast-foods. Folic acid is essential to cell multiplication like in the case of a pregnant woman. We should eat something green as much as possible everyday.
White is also a beneficial phytochemical plant pigment. Think potatoes, cauliflower, radishes, etc.
I eat science everyday, do you?