If I were to guess I would say that you planted a yellow variety cucumber. If you have tasted one and it is bitter you left it on the vine too long. Overwatering can also cause yellowing but it also affects the plant. There is nothing wrong with your plant. Another reason a cucumber can yellow is because of lack of trace minerals, but, again your plant is fine. A virus can turn cucumbers yellow but not an even color like yours and a virus also affects the plants. Yours isn't affected.