This blanket is joined in a two step process. Each weavie is finished with a row of single crochet in the same color as the weaving. The pieces are assembled using a whipstitch. When I whipstitch two different colored weavies together, I either use a completely separate color-- if that will add to the composition, or as in this case, I use one of the one of the two colors from the weaving. My rule for choosing which color to use is, whichever color you want to emphasize, use the other color to complete the whipstitch.
This afghan was made entirely with Red Heart yarns, which was all I could afford at the time and offered the variation of colors that I needed. Making an afghan with a single yarn type and manufacturer lets it go together a little better. The cloth seems to lay more evenly. However, that asset is offset by the differences in texture and design that can be captured by the use of more than one yarn type, which is very important to me.