The only way to do it is to use conditional formatting on, say, a rectangle object placed behind your controls on your form. In the query on which you base your subform include a calculated field whose value is, say, True if your condition that you need to test is satisfied and false otherwise, then use conditional formatting to change the background colour of the rectangle when the calculated field has the value True.
Reason you can't do it for the normal background of the form is that continuous forms comprise just one background and set of controls, repeated many times to display the rows (as explained in passing in NeoPa's article
Why Values in Unbound Form Controls Don't Persist).
-Stewart
