Those washing instructions are there mostly to protect the manufacturer! Once it's mine I can see fit to wash it at any temperature. If it's really 100% cotton and the dye is good, you should be able to do 60C.
My sheets go in at 90C (I'm old fashioned) but they are all off-white.
@CelloMomOnCars it shrinks more at 60°C than at 40°C though, even if mostly once.