>What makes this an invalid expression?
>
>This.SetAll('DynamicBackColor', 'IIF(checknum = 0, RGB(255,0,0), IIF(amount > 500, RGB(255,0,255), RGB(255,255,255)))', 'column')
>
>How should I set the backcolor to two colors based on two different conditions?

PMFJI, but I copied the code from your question and pasted it into a test grid and it worked fine. Something else must be causing the Invalid Expression error.