...because i need to split a polygon and i want to see the layer underneath it. But I have to select the polygon in order to cut it, and when i do that it turns yellow!
In version 1.7, go to Preferences > Options > General tab (opened by default). Then, click the yellow color in front of Selection color and change the alpha canal value.
My Qgis version is in French, sorry if the translation is not exactly correct.