Why so many fruits/veg fall into these color categories?
There are 3 primary colors, that being red, yellow, and blue. Most foods are red and yellowish due to the fact that blue is very rare in nature. Here is an article you can check out Why is the colour blue so rare in nature? 
Now, you might ask, why is green so common then if green is a combination of blue and yellow, and blue is so rare? The pigmentation for green is usually chlorophyll, which is an important pigment that plants use in photosynthesis. That's why it is so common in leaves and other parts of the plant as well.