I’d like to show my list of colours for vowels and letters, which I have always seen withouth will or effort, by thinking or writing them.
A: gray white, B: orange yellow, C: dark grey with violet hue, D: light grey with blue hue, E: middle green, F: dark violet blue, G: light blue, H: very clear grey, I: pure yellow, J: clear red, K: dark violet, L: light yellow, M: ultramarine blue, N: sky blue, O: brilliant white, P: marfil white, Q: grey white, R: red brown, S: golden yellow, T: wood-red, U: orange, V: yellow orange, W: intense orange, X: dark grey, Y: golden oker, Z: difused white, 1: clear grey, 2: yellow, 3: brown-red, 4: light yellow, 5: dark violet blue, 6: light blue grey, 7: red-wood, 8: dark brown, 9: orange, 0: brilliant white

And yes, as Kandisnky said, for sure a synesthetic, metal instrumets have an evident brilliant red colour, strings tend to yellow and green (violas, celos), and wood instruments can reach orange, very pure and clean colours. Drums don’t create colour, but vibrations of greys. Femenine voices tend to warm colours, but they are not as vibrating and visible as the sounds of birds. Some perfums can reach blue or green hues.