In English you can use paired adjectives (coordinate adjectives/cumulative adjectives) and most people do it a lot but what you have here isn't a group of paired adjectives.
If you wish to say that your car is red only when it's clean then the obvious phrase is:
- My car is red (only) when it's clean.
However, even this is practically illogical since the paint on your car doesn't change as it gets dirtier. I'm not a native speaker but I'd say something like this instead:
- My car is juicy red when it's not dirty.
As for the soup, you should say:
- This soup is best when it's scalding.
You can use the reduced clause and omit "it is" in either example:
- My car is red when clean.
- This soup is best when scalding.
This will provide you with the shortest possible way of saying that you want.