Welcome to the forum, Idefix.
Are you talking about the week view from shadow4 when you say "white colored today", right? First I suppose that you were talking about the calendar only.
If you don't like the colors and want to edit them, you need to unzip the Shadow4.r2skin file and edit the /images/grid-bg-*.png files (today and weekend, in your case) with any app which supports alpha channel, and then zip all again. Keep a copy of the files to avoid doing this again if the skin is updated or something like that.