It might be a cost issue rather than simple colors. A stain will most likely last longer without re-staining, compared to the white painting of the older coasters.
Consider this, most people (GP) consider woodies as "old and rickety" despite the coaster's age. Add to that some white paint that flakes off after a harsh winter. End result is people might think them to be older than they are, and that they are not well kept.
For example, Colossus at SFMM is white and looks great, from a distance. Once you are close to it, the flaking paint shows through.
It might be cool, however, if some woodies were painted crazy colors, like bright red or green. (I have not ever heared of one)
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Gotta ride 'em all!