Just to clarify this a bit further: と does not belong to から here. Instead, the sentence should be destructured as:
ピーマンはうれると - When bellpeppers get ripe (と here means “if”, but usually with a sense of “it is a natural consequence”, “it cannot be controlled by the speaker” or “it happens every time”)
みどり色から - from green
赤色になります。- they become red (になる in this case indicates a change, so this is where から comes into play because it marks the previous state).
(Disclaimer: That’s my current understanding. Please everybody correct me if I made any mistakes!)