Aperture’s Search Dialog allows you to select multiple color labels at a time. In other words you can filter out not only red labeled images, but add any other colors as well. It may not be obvious, but it’s there.
Select as Many Labels as You Want
Notice that I have the Red, Yellow and Green labels selected in the search dialog. It’s difficult to see that they are in fact selected, but they are, and you can pick as many of them as you like. If it’s not working for you be sure that the first clear button is not selected. If it is, all the images will appear and it won’t filter out the color labels. Also make sure the setting at the top of the Filter is set to All.