If I imagine a car, I can imagine its features, and of course color being a quite salient feature I would probably assign it a color (e.g. I could imagine a red Ferrari, or a black limo). It's just that I wouldn't see it, there wouldn't be anything in my mind similar to the actual experience of seeing a red object, I would just think about the "concept" of the car being red (hard to explain).

So far I've never found a way in which aphantasia really manifests externally or can be measured externally in a more or less reliable way. Which is why I'm still not 100% sure that people who claim not to be aphantasiacs aren't just exaggerating or taking metaphors too literally...