If the subform is a data sheet I think the answer is, you cannot, so what is it? Then what's supposed to happen if the box is unchecked (no one ever seems to think about that in these requests)?
Use the checkbox AfterUpdate event to set the subform detail colour property. If the control is True (you don't need the Value property) then change the colour.
The subform reference would be
Code:
Forms!YourFormName.YourSubformCONTROLname.Form.Detail.Backcolor =
NOTE subformcontrolname is the name of your subform control which not necessarily the name of either form.
To restore the colour if box is unchecked you'd need to know what it was before while testing for both True and False and acting accordingly.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.