Ah, yeah, the course hasn't done a good job of explaining this. Swahili has three basic colour adjectives:

-eusi = black -eupe = white
-ekundu = red

They carry prefixes that relate to the class of the noun they describe.

gari jekundu = a red car
magari mekundu = red cars
kitanda cheupe = a white bed
vitanda vyeupe = white beds
ndege nyeusi = black aeroplane(s)
ndege mweusi = a black bird
ndege weusi = black birds

Other colours are indicated by nouns, and you generally link these to the noun you're describing with -a. The -a has prefixes for the noun class, but the colour noun itself does not change. Sometimes you can place a noun directly after another noun without -a, but it's not as common.

gari la kijivu = a grey car
magari ya kijivu = grey cars
ndege ya buluu = a blue aeroplane
ndege za buluu = blue aeroplanes
ndege wa buluu = (a) blue bird(s)

By the way, if you need to use -eusi, -eupe or -ekundu as nouns, give them the ny- prefix ... but don't precede them with -a. E.g. Rangi ninayoipenda zaidi ni nyekundu / buluu. = My favourite colour is red / blue.