Can anyone help with an expression to alternate the colors between records under ‘Service’. The report is GROUPED BY ‘Service’.
I have tried =IIF(RowNumber(Nothing) Mod 2, "Silver", "White") previously however I think the GROUP BY is causing an issue here and cannot find a clear solution.

Use:
Set this expression in that box in your SS and you should be fine.
If you want to go even more in-depth and get crazy with your coloring, check out this blog post on advanced grouping and coloring.