Does the mutation here only apply to the noun car and not to the accompanying adjective glas ?
Yes. car 'catches' that particular mutation. glas mutates or not depending on the gender of the noun it is describing:
- Mae gen i gadair (I have a chair)
cadair las (a blue chair - glas -> las because cadair is feminine)
Mae gen i gadair las - two mutations, but for two separate reasons.
Very good, thank you.