I think it differs with different people. What I do if I want a controlled scrappy is to chose some colors that I know look good together. IE:, red and blue, pinks and greens, purple and green, orange and yellows. Now add in a third color that to your eye looks good. To the pink and green family I might add some purples, to the orange and yellow group I might add some green. So on and so forth. In the end some of my controlled scrappys end up looking very scrappy, depends on my mood and the fabrics I have.
You can also stick with the same color family, just get plenty of variety and pattern scale and hue.
If your worried about a certain color combo just photo it and put it on this board, the members all have good advice.
Remember to have fun!!