Your understanding is not correct. The on'yomi せい and しょう were originally from the Chinese pronunciations of 青. On the other hand, there had already existed the Japanese origin word あお, which meant "blue", before the kanji 青 was introduced from China, and later, the reading あお was applied to the Chinese character 青 which meant "blue" in Chinese. This is kun'yomi. There are countless kun'yomi nouns (あお is a noun in the first place), and on'yomi verbs (mostly -suru verbs)/adjectives.
Kanji - Readings - On'yomi (Sino-Japanese reading) - Wikipedia
Kanji - Readings - Kun'yomi (native reading) - Wikipedia
As often pointed out, you need to learn the language. To learn grammar is incomparably important than to learn each kanji individually. You should grasp the functions of が or を first, otherwise you even can't understand the meaning of the explanations of kanji such like 木を見るようにむかいあう for 相, right?