The red coloring is used as a friendly warning sign that this particular project isn't performing as it should. We conclude this by looking at the originally estimated budget and timeline, and then use two degrees of red to indicate a project is missing its estimated budget by a bit or by much.
The formulas are:
index = (budget spent% - work complete %) * (now - start / duration)
index > 25 = "WARN" (slightly red)
index > 75 = "CRITICAL" (very red).