Georgia asked for more tips on tension when doing color knitting. A certain amount of puckering can be expected - blocking really does work wonders in this situation. There are those nice times I have no puckering whatsoever and don't even need to block at all but then there are times when the puckering is hard to ignore.
On the tam I noticed some puckering while working with Pony Pearl dpns and Shetland yarn. I switched to two metal circulars and it went away. Dpns do seem to cause more problems for me and I really think some times problems in tension have to do with the particular combination of yarn and needles.