I have also seen it with bold purple, brown, and black streaks. These and the green colorations are caused by decay (which is typically not significant enough to cause any structural problems). That's probably why your co-worker told you you could get these colors by exposing the wood to the elements, although I doubt if you would get anything but warped boards.

In the Midwest, Lowe's Home Improvement Warehouses have a nice stock of milled poplar. Our local store in Champaign, IL typically has a decent supply of odd colored grains.

: George: I work with poplar on a regular basis at my job and find it
: interesting because of the wide range of color. What we get ranges from
: pink to brown to yellow to green but is mostly creamy white. A co-worker
: tried to tell me you could get these colors by leaving the piece out in
: the weather. My experience and intuition make me doubt this. Have you any
: knowledge of this? I'm thinking that a very striking boat could result if
: a guy could come up with a lot of one of the rarer colors. Thanks, Tom