I compare the 3-Color Coleto to the 2-Color Coleto after the jump. [Edit: jumps are silly; removed it]
The 2-Color pen body is a little thinner than the 3-Color body (note that the 3-Color body is still easy to grip and comfortable to use; it is not too bulky or fat). I don't own any calipers, and don't know the critical dimension (i.e., pen body diameter at the grip section) of either version of the Coleto. Sorry. What I do have is a couple of side-by-side pictures of the two versions. The size difference is noticeable in the pictures.
|3-Color (Left); 2-Color (Right)|
|2-Color (Left); 3-Color (Right)|
By the way, I'm pretty sure that the Coleto gel pen refills are available in 15 different colors and in three different sizes (0.3 mm, 0.4 mm, and 0.5 mm). There are also mechanical pencil refills, an eraser refill, and a stylus refill. This impressive assortment of refills opens up a huge universe of potential combos. Moreover, the Coleto line includes several different styles of pen bodies, including 2, 3, 4, and 5 color versions. I might try a 4-Color version, but am concerned about the girth of the grip section.
PS - Kids love these multipens, too! Look at the green and red Christmas colors in the above picture. Nudge nudge, wink wink.