I understand your reasoning, but it is not very logical. It depends on what you mean by "imagine a triangle". A triangle doesn't have to be filled with a color to be a triangle. Why would you imagine it with any color in it in the first place, not just a wireframe, being transparent inside? So if your triangle doesn't have a color, it might be because you are more accurate in your imagination, not because you have no triangle imagination.

So without a proper way of defining things like "imagine" or "mental picture" first, tests like yours or like VVIQ are not well-defined. Now, maybe I don't understand the meaning of VVIQ because I have aphantasia. Or maybe I don't understand the meaning of VVIQ because I am a logician and mathematician, and I am very peculiar about "meaning". Maybe even both. Without a doubt though, VVIQ is not an objective test, because its elements are not properly defined.