Hi LC
Highlight all cells you want to be affected by the condition
Click on Format/Conditional Formatting
Cell value between 0 and 255, set your formats, Click Add
Cell value greater than 255, set your formats, Click Add
Click OK

HTH
Michael

"LC" wrote:

I would like to have the background of cell automatically shaded in grey
level based on the value contained in the cell (values between 0-255).
0 would correspond to Black
255 to white