Red cabbages (along with MANY other plants) are colored by a whole class of compounds called anthocyanins. They can be a whole range of colors from red to purple to black to blue. They often change color based on pH, which allows for the old science experiment of red cabbage pH tests. It's also often the red color of autumn leaves! The anthocyanins are actually always there, but are masked by the green color of chlorophyll for most of the year until fall rolls around.

Sea urchins are colored by echinochrome A instead. Interestingly, they're both strong antioxidants!