The words grayer, greyer sound the same but have different meanings and spellings. Why do grayer, greyer sound the same even though they are completely different words?
The answer is simple: grayer, greyer are homophones of the English language.
comparative form of gray: more gray
comparative form of grey: more grey
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").