Computer - Hard Drives Icons - Change Free Space Colour

Hi. I've spent hours trying to find this answer so thanks if you can help.
When on the "computer" window and icons set as tiles, hard drives report
"free space". I like this view.
Usually showing blue but changes to red when free space is 10% or less. I
wish to change when Vista considers my hard drives are nearly full (ie the
10% default setting).
As an example, one of my storage hard drives is 300GB with 22.7GB free; I
don't consider this neads flagging but Vista does. I wish Vista to reflect my
threshhold for reporting.
Where is the "hidden" switch to alter how Vista report this please?
Thanks,
soup