Synesthesia is definitely real, my wife has it. She sees numbers and letters as having colours. We'd been married a few years before either of us realised there was something different about her, she just thought everyone saw the world as she did. I didn't believe her at first, then asked her go through the entire alphabet and tell me the colours she saw for each letter. To this day they've never differed.

Words written in coloured ink are difficult for her to read, and trying to read "rainbow" coloured text is virtually impossible as it makes her feel sick.