>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?

Comparing it to this example from a help screen

Form1.Grid1.SetAll("BackColor", RGB(255, 0, 0), "Column")

I don't think your second arguement is supposed to be character.

This.SetAll('DynamicBackColor', IIF(checknum = 0, RGB(255,0,0), IIF(amount > 500, RGB(255,0,255), RGB(255,255,255))), 'column')