When you design you have to make choices. One of them is am I doing a controlled look or a totally scrappy look. it is just something you can choose. I am more of a controlled scrappy person. I prefer a light dark situation so I can get a distinct design to show up. I use light med and dark to get the looks I want. I like lots of contrast to make the pattern pop. If you use meds together and lights together etc then your sections will blend and you wont' have a specific design. But that is a choice to make. Either one is great just a diff look.
One thing I like to do when I want to do scrappy but keep it controlled is to use colors that are opposite so they each stand out. so I'd use reds and creams and maybe a blue or green. Then all my reds would be diff fabs. and all the creams diff fabrics etc. that way I am controlling the pattern but still making it scrappy.
I rarely do anything that is totally scrappy with no thought to how and why the pieces are placed. Even my confusion(my version of crazy quilting) is somewhat planned.