Interesting topic! I visualize numbers and letters and scenes exactly like you presented. I also dream very vividly, except for the sense of smell, my other senses seem to become super acute. Emotions and the sense of 'atmosphere' are heightened to an almost ridiculous degree, which I think is a sign of some form of mental problem since while awake, I usually feel very bland and dull, almost emotionless.
I find the coloring of letters most interesting, I don't have synesthesia in the literal sense, but I do assign colors to each letter and number.
Hard to explain, but when I see a number or a letter, I see a 'flash' of color in my minds eye, sequences and words appear as a gradient or mix of
the individual colors. My favourite numbers, 6 and 9 also correspond to my favourite colors aswell, 6 appears as an electrical, vibrant shade of blue,
like the current ATS background, while 9 is a deep violet. Mixed (69 ), they form a beautiful, tranquil gradient.
I've tried to figure why this happens, I presume it's a memory related thing. Tying abstract symbols to something more familiar and intuitive like
colors. One possibility, remember the alphabet books? I recall they often had the letters uniquely colored, hmm...
Would be interesting to know how many people experience this too.