After messing around for decades trying four black squares spaced different distances apart, black square with white center, single square, diamonds, circles, white, black, orange and some unremarkable combinations I can’t remember, the end result based on group size and ease of use is a solid black 2 MOA square for a 6x scope. My shooting/hunting buddy likes orange dots and high powered scopes, but shot the smallest group of his life with my setup in a really accurate 7 mag. It amazed him to put 5 shots at 200 yards into less than an inch time and time again with such low magnification and a “giant” square.
With a 12x scope I’ll half the size, and 20x is 1/4 the size. One summer after getting a printer that worked with card stock, I printed squares in 1/2 MOA increments to see what size did to groups. 2-1/2 MOA or 3 MOA squares also worked, but seemed to require more effort to keep aligned. 1-1/2 MOA or even 1-3/4 MOA were just too small.
For whatever reason the small amount of the square poking out on all sides of the reticle are easy for me to center up on. White on a black background was no better, so as a practical matter I’ve stuck with black.