1. It's kind of like humming or singing, just internally.

2. Only via memory recall and even then its not stable and I don't "see it" with what my eyes are seeing/processing but more with the memory/recollection part of the brain. As such I can't visualize a unique image of that, but can construct one internally, often recalling different memories/images of the various parts of what I'm constructing.

3. No. I can't visualize (or really "see") colours on their own. When visualizing a colour I recall things that are that colour from memory (an apple, grass from a particular place, the Windows XP background, etc.). It's not that I don't know/remember that something is a specific colour, just that that part is not recalled/imagined visually. Or when I do recall colours of objects/pictures I can't hold onto that image.