Synonyms may help people to figure out meanings by known words. If a word's meaning is unknown, atleast one of the synonyms must be a known word. So, the meaning of the unknown word can be explained by the meaning of the known synonym, as a synonym is an alternative word with the same meaning.
This is incorrect. Gouache painting is very very similar to watercolours, so much so that the two can be used together. It is an opaque method of painting.