Can "some" and "certain" be used interchangeably in this example? Are they perfect synonyms in the example?
Some cars are red.
Certain cars are red.
PS: There is a similar thread but it does not answer my question
"Certain" in this context is another way of saying specific.
So, saying "certain cars are red" would beg the question "which ones?" You would need to be able to specify what other factor(s) determine whether a car is red or not in order to say this. I'm not personally aware of any make/model of car that comes exclusively in red. And most cars come in red as an option. Nothing else really determines the colour of a car other than the final choice of what paint to spray the bodywork with, so it doesn't really make sense to say this.
"Some cars are red" is perfectly true. Some are red, some are blue, some are green, and a whole host of other colours.