For a grapheme-color synesthete, the character 5 might always be yellow and the letter A always red, whatever ink they are printed in. The colors are specific, stable across years, and differ from person to person.
This form dominates the research literature because it tests cleanly: show graphemes, collect colors, retest months later, and measure the drift. Genuine synesthetes show far tighter consistency than controls paid to memorize matches.
Grapheme-color pairing often coexists with other forms. A person with colored letters has elevated odds of colored sounds too, which is one reason researchers treat synesthesia as a family of traits with a shared mechanism rather than isolated quirks.