Are you using a continuous form or a datasheet?
Continuous forms are a little more malleable. I tested this with a continuous form:
in the ON CURRENT event of the subform use this code:
Code:
If Forms!<main form name>!<field name> = <value> Then
Me.Detail.BackColor = vbRed
Else
Me.Detail.BackColor = vbGreen
End If
Where main form name is the name of your main form
field name is the name of the field with the value you want to determine conditional formatting
value is the value you're testing to see whether or not you want to apply the coloring.