Those three colors have their own instruction, separate from the infection list, and it is short: contact your provider if your foot ulcer is very white, blue, or black.
It stands on its own, apart from the signs of infection. That is the point: you are not being asked to decide whether the ulcer is infected. You are being asked to report a color.
A healthy wound bed has a color too. After a debridement the ulcer may seem bigger and deeper, and it should be red or pink. Wounds that are pale, purple, or black are less likely to heal.
That is why color is worth reporting rather than watching. What a particular color means in your foot is something only an exam can tell you.
Ischemic ulcers, the wounds caused by poor arterial blood flow, often occur alongside diabetes and nerve damage. Those sores can be dark red, yellow, gray, or black, and the leg can turn pale and cool to touch when it is raised. Poor blood supply and color change are connected.
One thing makes this harder than it should be: the ulcer may not hurt. Diabetic ulcers are often painless, because of decreased feeling in the feet, so color may be the only thing that has changed.