You can't do it via the user interface, but you can do it in VBA code.
The text box or combo box control has a FormatConditions property, which returns a reference to a collection of FormatCondition objects. Each FormatCondition object has BackColor and ForeColor properties you can set to any color you like.
Assuming you want to save the form with the custom colors set, you'll need to write a routine that you can run at design time, from the Immediate Window. The form must be open in design view when you run the procedure. After you run it (once for each condition, for each control), save the form. You could then delete the procedure, since the colors have been saved with the form design, though you might want to keep it around for reuse later.
Here's a simple procedure that should do the job. (I haven't tested it, though.)
Code:
Public Sub SetFCColor(FormName As String, _
ControlName As String, _
Condition As Integer, _
BackColor As Integer, _
Optional ForeColor As Integer = 0)
Dim ctl As Control
Dim fmc As FormatCondition
Set ctl = Forms(FormName).Controls(ControlName)
Select Case ctl.ControlType
Case acComboBox, acTextBox
Set fmc = ctl.FormatConditions(Condition)
fmc.BackColor = BackColor
fmc.ForeColor = ForeColor
Case Else
MsgBox ControlName & " is not a text box or combo box"
End Select
End Sub
Copy this code and paste it in a new module. Then open your form in design mode, add the conditions (with any old colors) if you haven't already, and switch to the Immediate Window. For each condition on each control that you want to change, enter the following:
SetFCColor <name of form>, <name of control>, <condition number>, <back color>, <fore color>
The condition number 0 refers to the default colors. Conditions 1 through n refer to the first, second, etc. conditions you have entered.
For the back color and fore color values, you can either specify the numerical value, or you can enter the RGB() function that computes the numerical value from the red, green, and blue components, each of which is a level from 0 to 255. To get a numerical value for a color, you might set the plain old Back Color property of a form using the Build button, and then copy the number that pops up in the property sheet to use in the Immediate Window.
You can omit the fore color when you issue the SetFCColor calls in the Immediate Window; if you do, the color of the text will be set to 0 (black). Rick Sprague