I have seen many monochromatic string quilts and scrap quilts. I think they look great. Google images of "monochromatic string quilt" and you will see some. I actually prefer controlled scrappy to just put my hand in a bag and see what comes out and use it. I am always happier with the end product if there is some semblance of order. Another option is to go with one color family like all cool colors (blues greens purples) or all warm colors (reds, oranges, yellows). Or one theme, like all novelty kid prints or all civil war repros or all batiks. I do better with color coordination than I do with theme and sometimes I combine the two. Like this quilt was all asian scraps and I sorted the logs in each offset log cabin to be all in the blue family or red family or black family and the lights I just used lights. There were a couple of all out scrap ones in the mix but they were all Asian prints.
Scrappy log cabin clam shell setting