I have a spreadsheet that contains some cells that are manually colored (not
by conditional formatting). However, I want to run a conditional format on
the whole table but only have it apply to cells that are not colored already.
Can this be done? Here is a quick example:

Name Jan Feb Mar
Smith 150 215 195
Johnson 0 145 110
Fisher 125 95 115
Sweeney 165 125 210
Wright 110 130 140
Dunn 25 150 120
James 65 130 105

The cells for Johnson for Jan & Feb and the cell for James Mar are already
highlighted by me (in yellow) for other purposes. I want to do say Red at 75
or less, green at 76-125, and orange 126+. However, I do not want to change
my yellow cells only the cells that do not have a background color now. Can
this be done with conditional formatting? Thanks