Another option is to write a function to do this and an example below involves a case statement. In the color property of the textbox, you can reference the function like so:
If you had multiple textboxes on a report that required the same colour formatting, this technique would be a bit more reusable.
Public Function GetColour(ByVal InputValue As Double) As String
GetColour = "Red"
GetColour = "Blue"
GetColour = "Green"
GetColour = "Black"