Because a table is not a colour. A table is 'of' some colour, not a colour itself.
For some reason the distinction exists only in statements in English (the table is of the red colour), but not in question (what colour is the table? — of disappeared somehow), but Russian distinguishes these constructions everywhere.
This is a different grammatical construction, it would be translated as «Како́й цве́т э́того стола́?» or «Како́в цве́т э́того стола́?».
According to Vasmer's etymological dictionary, those words are in fact related, both originally meaning something standing upright (related to стоять).
Also, according to Slovar slovenskega knjižnega jezika, Slovene 'stol' can mean 'table' in some dialects:
stòl stôla [ou̯] m, mest. ed. tudi stólu (ȍ ó) 1. kos pohištva navadno z naslonjalom, namenjen za sedenje ene osebe [...] 2. nar. miza: stol je bil pripravljen za kosilo; sedeli so okoli stola [...]
The Russian g has me confused. (Sorry, can't figure out how to get Cyrillic letters without changing my phone's language.). In the first word in this sentence, in the Duolingo sound file, it sounds like a v. (And in many others.). In other words it sounds like an l, and I hit a word last night in which it sounded like an English flat r... And a few days ago I actually encountered my first word in which it actually sounded like a g.
What is going on here? Is my phone's sound just that bad? Are the sound files that indistinct? Or is it just that no, really, it just very rarely makes an actual g sound when in a word?
One sad aspect of the mobile version of DL is that it seems not to give access to the tips or hints section at the beginning of each section (I hadn't realized that it gave access to comments, either--maybe it is a difference between the free and plus versions, or maybe I am just incompetent with my iPad). Early on in the desktop version of things it was mentioned that г is often pronounced as a g sound, but typically in endings such as его or ого it has more the English v sound, just as if it were a в. Don't know about the l or r sounds you have been hearing, though.