People that see sounds as colors and shapes or hear colors as sounds, see numbers and letters as certain colors. There was one lady who saw musical notes as man-sized complex 3D shapes and a boy who couldn't see the board in class because someone's voice turned his field ov vision full of black blobs. Also twins who both associated letters and numbers to distinct colors, but completely different ones.
Host distantly knew someone in school who saw colorful letters, apparently it's 4% of the population so quite a lot. I guess it's vaguely related to tulpamancy, we try to induce pseudohallucinations and some people report being really successful at it. I believe them. Best thing we achieved was 'feeling certain colors with our rubik's cube imposition experiment. Imposed a 'completed' rubik's cube and rotated it to see the differently colored faces of the cube. I think most people can achieve seeing vague colored shapes with enough dedication.
But synesthesia also includes weaker forms of people doing what tulpamancers do and impose certain concepts like calenders. Ok, we do that too and have a yearly and lifelong calender that can be imposed and searched. Isn't this normal? People who have no inner monologue and are unable to visualize objects and abstract concepts are abnormal.