Kelly's right. You need to understand that k and p are opposite of each other. That is, the back of a k st is a p and vice-versa. So whether you knit every row or purl every row, you're going to end up with the same thing. If you knit one row and purl one row all the way up your scarf, you'll get what's called stockinette st, which is what we are used to seeing on the outsides of sweaters. It does tend to curl, though, so you could knit 5 st on every row for the first and last 5 st of each row, whether it is a knit or purl row.
The bumps of different color occur on the purl side when you change colors. On the knit side they look like a smooth transition, though.