There should be a comma.  It depends on whether the relative clause is restrictive or non-restrictive.

Restrictive clauses are used to identify something/someone.  They have no comma.

I took the ship which was red.  (In this case there would have been many ships but only one was red, so  the ... which...  identifies it or 'restricts' it to that particular ship).

Non-restrictive clauses just add information about something/someone (when that something/someone has already been identified or when they are already familiar to everyone).  They have a comma.

In your example, red is a colour that is already familiar to everyone.  You aren't identifying which colour it is, you are just adding extra information about it (= you like that colour).

Another example would be:

I took the cheapest ship, which was red.  (In this case, the ship has already been identified (as the cheapest one) and you are just adding extra information (= that it is also the colour red)).

Sometimes it is hard to tell and it is only the context and/or the comma (if in written English) that makes it clear.  For example, the first sentence I gave as an example could be a restrictive clause in a different situation:

I took the ship, which was red.  (In this case there would have been only one ship, so there's no need to identify it any further and the extra clause is just to add information).

Note - My examples are contrived, purely to exemplify the difference between the two types of clauses.  A bit more research on usage would be needed to decide when to use a restrictive relative clause in the first place (eg. it would be clearer to say "I took the red ship" than my first example).