Thx for all the answers.

set_num_format can only be used to numbers (like the method name says it ;-) but I have to compare strings. And even if I replace high, medium and low with 1,0,-1 it only changes the font not the background color...

Unfortunately Win32::OLE can only be used on Windows and I'm using Linux... So I have to look for another solutions. Thx anyway, and if I find something to solve my problem I will post it.