This is one of those cases where it is very difficult to give a general rule. I will try to give a good explanation. There is also a reference to another explanation at the end of this comment.
Normally, you can use either "except" or "except for".
When to use only "except for"
When you refer to the properties of something and you want to exclude one of the properties, you use "except for".
In your sentence, the property of colour is being excluded: I like the car very much except for its colour.
Let us look at another sentence: The book is very good except for some grammar errors.
Another example: It is a wonderful country except for its president.
When to use only "except"
You only use "except" before a conjunction or another preposition.
Conjunction: I am happy except when it rains.
Preposition: He sleeps everywhere except in his bed.
In some cases, you only use "except" before a verb. It depends on the context.
1. She does nothing except eat. (does ... eat)
2. They are interested in nothing except talking. (interested in ... talking)