The easiest way to do this is to format a cell the way you want it, then use the "cell format ..." contextual menu to get to the fill and format colours, use the "more colors ..." button to get to the hexagon colour selector, select the custom tab.
The RGB colours are as in the table at the bottom of the pane. If you prefer HSL values change the color model from RGB to HSL. I have used this to change the saturation on my bad cells. A higher luminosity gives a worse results and the shade of all the cells is the same just the deepness of the colour is modified.