Take any given color, for example, yellow. then go to the color across from it in the color wheel, in this case it is purple. These colors, called complementary colors, go well together, or complement each other . Complementary colors usually are good when choosing your block palette.
Now usually after I choose a pair of complementary colors, I go for analogous colors, or colors right next to each other on the color wheel. One could say that since every color is next to each other then each color is analogous, but if you used every color in a build, it would look like puke. So try to use analogous colors in a 3 color radius.
After choosing colors, its time to choose grays, grays are colors ranging from white all the way to black. Using grays are always helpful, but not to be overused, otherwise the build will look dull. Typically I will pick 2-4 different grays and then just use depth to get the right gray. If we are sticking with our original yellow+purple combo, white would be the best gray to use, for black would blend with the colors, where as a white or light gray would make the colors pop.
But seriously, block palettes are more a trial and error sort of thing, you can't really get it right on the first go, it takes me as long as 45 minutes to get my palette right, even then I am constantly adding blocks to my palette as long as taking them away. Hope this helped mate.
