I've been playing with getting a tweed look by using two different colored strands together. I found it easy to make a smooth appearance. First, I don't let the two strands twist when I wrap the row, so if the dark color is on top, it stays on top for the entire row.
Second, I make the colors sit next to each other when wrapping a row. So, if the light color is on top for the loop already on the peg, the light color is on the bottom for the new loop being created. On the next row, the dark color is on top for the loop already on the peg, so the dark color is on the bottom for the new loop being created.
By letting the yarn twist as it will, you will still get a tweed look, but it will have darker and lighter sections at random. This is perfectly fine also; it just depends on what look you are going for.