That might be by design if you turn the monitor on without turning on the PC, laptop or whatever it is connected to, although usually it's black then to save energy.
Since it doesn't make much sense to turn it on when there's nothing to look at, it's easy: don't do it.

If it's white when you expect it to actually show something (you didn't say that, but just in case) the test is to try it with another PC or laptop as source. If it's the same there, check or replace the cable or have the monitor repaired or replace it. If it's OK on another PC, the suspection is on the graphic card. Confirm this suspicion by testing with another monitor.