Use conditional formatting for this.
Enter layout mode.
Select your field or fields where you want this and select Conditional... from the format menu.
add a series of conditonal format expressions such as:
MarksField > 70
MarksField > 80
MarksField > 90
Select a different Fill color for each expression.
The expressions are evaluated from top to bottom so the last true expression will select the color for your field or group of fields.
thank you "PhilModJunk" this works great and so much easier than I thought it would be!