I always buy exactly the same underwear - Fruit of the Loom boxer briefs "trunks". The trunks version have shorter legs and allow me to wear them with skinny jeans. I buy them in the Black/Grey pack which has 3 of each color so all my underwear are exactly the same and in only 2 colors.
DAE do something like this? I find it works perfectly for the way my brain works. I also only buy the exact same socks (excluding my dress socks). I buy 1 version of long socks for the winter and a version of shorter socks for the summer so that anytime i pick out socks I never have to worry about "matching" them up. It is either a long or a short sock. Makes things easier.
For me it's always Fruit of the Loom boxer briefs with the longer leg cut. Normally in blues/grays/greens. For socks I have ankle-length white and ankle-length athletic black socks. I always buy the same brand socks (they're also Fruit of the Loom because I hate that the Hanes say "HANES" on the toe) and I make sure to pair them up differently as many times as I can so I don't end up with a ton of socks leaning left or right.