You should use 'BackColor' property of the control for this.
First you must set 'Back style' property to 'Normal'
Then in report's 'On Open' property put a code like this
- Me!YourFieldName.BackColor = 255
This will turn that control's back color to RED making that entire column red.
Numbers of colors you can see if you go to that control in design view and click '...' in the 'Back color' property
You can use the Rectangle control that you put behind the field control with the same principle, but above described way is visualy looking better to me.
And you can combine this code with
If...Then statements to get different colors etc.
regards, H.