Okay, this is a little embarrassing to admit, but I'm a bit confused by the colours in Swedish. I assume that for every colour there's the basic form used for en-words (for example vit) and then you create the ett-words form by adding a "t" (vitt) and the plural by adding an "a" (vita). Does this work with every basic colour or are there any important exceptions? And what's the difference between röd and rött? I would assume that the latter is the form that goes with ett-words, but that's apparently not the case.
If anyone could answer these I'd be very grateful.
No, you’re right. The basic idea is that you add a -t to the neuter and -a in the plural as you say. There are some very minor irregularities though.
röd → rött (you cannot pronounce rödt so the d changes (assimilates) to the t and becomes tt. Rödt is actually the pre-1906 spelling however.)
blå, grå → blått, grått (adjectives ending on a vowel get a double -tt, and therefore a short vowel in the neuter)
blå, grå → blå(a), grå(a) (because of historical reasons, the -a in the plural is optional in these specific words, but it’s usually more common to have it nowadays)
Some adjectives are also indeclinable, so they don’t change at all, such as rosa (pink). Also remember that the -a is used not only for plurals, but also for the definite form, such as den röda boken (the red book).