To my knitters: How do I do what's in this picture? (and some other questions).

http://alison.knitsmiths.us/pattern_beginners_scarves.html
(this is the same link sdeutsch gave me in one of my other knitting questions)

Please look at the third picture – the “simple ribbed scarf”. See how it’s two different shades of blue? So, the pattern doesn’t say how to change between the different colors? How do I do that?

Also, it says to cast on 26 stitches and do 2×2, but last time I did it I did 26 stitches and then did 4×4 (and it turned out beautifully, thank you so much sdeutsch and other flutherers that helped me through it!!). So this time, I want to try 3×3. Can I just cast on 30 stitches, or is there anything else I need to know? (I know 27 is a closer multiple of 3, but this yarn is a bit thinner than the yarn I used for the first one).

Thanks!!