His Circuit Special is green and his GLA is grey, as is noted on the Wiki.
Super Glider uses the same shades of yellow and green as his standard kart, with yellow fill and green line.
As for all other parts that may be of interest and aren't on the wiki yet:
Bounder is grey, Koopa Clown is default green, Splat Buggy is default pink/lightblue
I haven't yet unlocked the Ink Striker of the bunch, but as that one doesn't change colour for non-Inklings, you can imagine what it looks like on Jr.
Getting a few screenshots for you as we speak.
Just reposting and deleting my previous one to make sure you see it and avoid double posting at the same time.
I've packed a bunch of character selection-screen screenshots in a ZIP file for you to go crazy with.
characterkartimages01.zip (19,5 MB)
(SHA1 is BF0A314584E0DCFD596FDD43070AD759E2047FD6 if you're paranoid)
Contains aforementioned Bowser Jr. shots, as well as Standard Quad and Bike pictures of the characters that didn't have 'em.