Synesthesia
Both Wesley Trisnadi and his elder sister were born with Synesthesia, a neurological phenomenon where the stimulation of one sense involuntarily triggers another. "Syn" translates to joined, while "Aesthesia" means senses, therefore "Syn+Aesthesia" means "Joined Senses".
While there are various forms of Synesthesia, he claims to have only 1 form (at least to his current knowledge), that is Grapheme-Color Synesthesia. His mind involuntarily corresponds to every letter and number with a specific color and opacity, which ultimately corresponds words and even names to multiple colors as well. He is not color-blind.
Back when he was 16, Wesley Trisnadi gave an official TEDxTalk of Synesthesia, when his high school hosted their annual TEDxJIS event.