the vehicles are so detailed in this game , i wish Rosalina's Circuit special was Princess Orange's and not the Boomerang one.
Rosalina's Circuit Special is colored blue, most likely because her dress is a lighter shade of blue (most notably, turquoise). A blue varnish never did exist for the Sport Bike, which is likely why Rosalina was given the yellow varnish.
However, Rosalina is a bit unique with her varnish color choices, since the characters who use the blue varnish for the Circuit Special, usually use the red varnish for the Sport Bike, which is not the case with Rosalina and Baby Rosalina. Of course, White Yoshi, White Shy Guy, Lakitu, and Ludwig use the green varnish for their Sport Bike, despite using the blue varnish for their Circuit Special.
On an additional note, Link's vehicles are never colored green at all, except for the Biddybuggy. The same thing also applies to the red Shy Guy, where outside of the Biddybuggy and Sport Bike, none of its vehicles are colored red.