A blue weekend is a completely free weekend. Cadets can leave after LMD (last class or retreat) on Friday, assuming they are not restricted and have passes.
A silver weekend is a training weekend. There is either a SAMI, some kind of military training, or a home football game. Depending on the type of training, Cadets may be able to leave on a day pass Friday. Then after training is finished on Saturday (usually by noon-1pm), Cadets are release and have the rest of Saturday and all of Sunday off.
I'd say probably about 2/3 of the weekends are blue, 1/3 silver, but I didn't actually tally up the weekends to check that for accuracy.
Also, once 4*s are not restricted after parents weekend, they have plenty of opportunities to get out. I did not use all of my passes either semester last year, and borrowing a car is usually possible. Granted, going out in service dress does get old, but 4*s do get out plenty if they want.