Knit-alongs are a great way to tie contrasting colors together, but they can also be expensive or draw too much attention away from the primary yarn. One thing I like to do is use size 10 crochet cotton along with my primary yarns. this gives a tweed effect to the fabric without detracting from the other colors and textures I'm using. It also avoids changing the gauge too much.
Another benefit to a neutral or coordinating knit-along is that it broadens your color choices for the primary yarn. Suddenly colors that seem to make too bold a contrast can be tied together to create a unified and intentional look. This is a great way to save money on yarn: if you can make diverse yarns in your stash go together, then you can rely more on your stash for larger projects, rather than having to buy new. Now, granted, buying new is a whole lot of fun, but it can be hard on the pocket book and time consuming when you have something particular in mind. It's also not terribly practical when you just have to start that new project at three a.m. on a Friday night. And even if you end up buying the cotton, it's one of the cheapest yarns out there.
And that's my two cents.