I have a black one I use for wash up water
all depends on the amount of sun hitting of course
compared to the tan not sure how much hotter never measured but its noticeable maybe %20 or more ? really cant say for sure
did leave the black one out here and was like 90+ degree day forgot how long but the water was HOT like uncomfortable hot !
while the tan one was just warm/hot !
my other ones are tan we use for drinking water we keep inside the tent usually
never had the dark green color ? but if you can get the blacks for wash and get some sun they truly heat up better than the tan