I am not an expert on the subject but i think it is because lighter colours absorb less heat than darker colours. Black absorbs more heat while white rejects the light without much absorption of heat and this is one of the reasons for lighter colour.

And if you ask why not white because you can see the dirt clearly on white and not on yellow.

Also polymers used to make tanks can be engineered to any colour yellow being bright and not visibly dirty would be the best choice.