White means two different things depending on whether it is the wound or the skin around it, and one of them has its own instruction to call.
The skin around the wound. This one is explained by its cause rather than its color. Keep the dressing and the skin around it dry, and try not to get healthy tissue around the wound too wet. Too much moisture can soften the healthy tissue, causing the wound to get bigger. Soft, pale, waterlogged skin around a wound is a dressing problem, and it is one that enlarges wounds.
The wound itself. This one is more concerning. Contact your provider if your foot ulcer is very white, blue, or black. That is its own instruction, apart from the signs of infection, and you don’t need to interpret it.
Also: wounds that are pale, purple, or black are less likely to heal — and after cleaning or debridement, the wound should be red or pink and look like fresh meat.
There is a third thing that can look white and is not either of those: dead tissue. Sores can be covered in dead skin that is yellow, tan, green, or brown, and slough and eschar are among the devitalized tissue that debridement removes.
So there are three possibilities, and where the white is tells them apart. Around the wound, on soft and soggy skin: a moisture problem. In the wound bed, pale rather than red or pink: something to report, and on a foot, its own instruction to call.
Wound Wizard has a separate article on skin around a wound that is white and soggy, which goes further into the first of these.