Access does not have a way to recognize Grouping in Forms in the UI. (it does in Reports)
However, there is an Event of the Detail Section that provides an appropriate hook for formatting, even without the UI "Grouping".
Since there is an integer index for the groups, and we want to alternate by group, it is easy to just check for even/odd groups and set the background colors.
We need to force both "Backcolor" and "AlternateBackColor". So assuming the group index is put in a textbox named IndexColumnBox, it can be as...
Private Sub Detail_Paint()
If Me.IndexColumnBox.Value Mod 2 = 0 Then
Detail.BackColor = &HDDDDDD
Detail.AlternateBackColor = &HDDDDDD
Detail.BackColor = &HFFFFFF
Detail.AlternateBackColor = &HFFFFFF
(Note the event is screen paint, and not just generating the Continuous Forms once. So this event will be called any number of times for each row as the Form is scrolled and used. This means it is key to have something like the index column check that is always the same.)