If I say "imagine a triangle" and ask you what color it is, why would it have a color if there is no image there? I didn't ask you to imagine a red triangle. Only a triangle. The only way it logically gets a color is if you are visualizing it.
This void is a 3D space. There is a general sense of up, down, left, right, front and back.
In this void I materialize a triangle. It is a wireframe. 3 lines, all the same length. No color, they are just lines.
By default the triangle is resting on one side, pointing up, and facing me. I can then rotate and move my mind "camera" around it while simultaneously rotating and moving the triangle as well.
The more complex the shapes, the more shapes, the harder it is to maintain coherency. I can close my eyes and concentrate to help.
I can assign colors if I wish to, but they are not really there visually. More like an extra information attached to the shape.
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.
> 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?
That’s just it: when I imagine a triangle, it is a definite image with a definite color. The visual details like color come automatically for me. I cannot imagine it in the more abstract way you are describing. If it’s a wireframe, the wireframe is gray. It’s always a full, colorful, 3d scene, perhaps vague but definitely with visual detail.
What else has this triangle got? A flavor? A smell? A name? It would have never occurred to me it had a color in the first place before you asked.
This does not follow. I can imagine a red triangle without visualizing one.