my beta fish is about a year and a half old. when I got it, it was a really pretty blue-green. Now it is changing colors on me. its belly is orange and black striped but its tail and fins are still the blue-green. What is going on?

no hes not sick its just the food changes their colors

Genetic Inheritance is very complex and can confound the best of scientists so do not spend too much time on it.
There are dominant genes and recessive genes. The partner with the dominant genes will show up in the offspring but the one with the recessive genes will not be so prominent. The result is usually like a throw of the dice. You never can tell what will be the result with certainty. Sometimes you get the most unusual species and the most spectacular results while sometimes with all the most careful planning and execution you still get very mediocre results. So just enjoy natures way of taking care of itself. imagine if all the world were of just one kind of human what a bore it would be.