What’s the difference between “my previous car was blue” vs “my previous car is blue”? Is one of them wrong? Or can both of them be right, being different only in the context?
Generally, you'd say "my previous car was blue," since you are likely talking about the color of the car at the time you owned it. Most people don't keep tabs on their old cars or their paint jobs, it would be somewhat unusual to state the color of a car that you haven't owned or seen in an indeterminate amount of time. I couldn't even tell you with certainty that any of my previous cars even still exist, much less what color they are. My previous car was blue, but these days it may be painted a different color or crushed into a cube.
You could say "my previous car is blue" if you are, in fact, talking about the color of your last car at the current point in time. If you went to a used car lot and saw your old vehicle for sale, you might point it out to someone by saying "my previous car is blue," since you can see the car and know what color it is right now.
Overall, the use of is/was depends on the point in time at which the car was blue. A car that was blue may not be today, and a car that is blue may not have been in the past. Which is preferable will depend on when you want to anchor the car's "blueness", which will typically be the period you were most familiar with it, in the past.
It's a bit strange in English to refer to your "previous car" with a present-tense verb. If you said the second, I might ask how you know it's still blue now — after all, the new owner might have had it repainted green – or why the color of your former car is important.
There are valid reasons to use the latter. Perhaps you are sending me instructions to go to the dealership because you just realized you left something in the glove box when you traded in for your new car and need me to be able to identify the old one. Or perhaps it was stolen and you are trying to identify it for the police. However, outside of this sort of situation, the former is preferrable.
First, both are right!
Another reason applied for @Nuclear Hoagie's is that you are telling the truth (not related to tenses at all, you don't mention whether the car is still existing or not, but just want to tell us the fact that your used car is red, so simple to understand).
A typical example is when you describe a very considerable person, we can usually see something like this following in a normal historical book:
Isaac Newton is a famous scientist....
Here we use "is" ("was" is also fine), because we want to explain the detailed information that he is a very famous scientist (that's the truth, and until now the truth is still alive), we've ignored the tense whether he died or not.