Some nouns can take one ending to become an adjective or another ending to become an adverb. Not all nouns can form an adverb and an adjective. Some form neither of the two. The rules aren't precise here.
Like in English, adjectives precede the nouns which they describe, regardless of what place in the sentence word order the word falls.
Why would there be a need to make words like "green-ly" and "blueness"? Well, to start with, நீலம் is the word for blue. In Thamil, colors are nouns, not adjectives.
In simple sentences in which something is being described by an adjective, adverbs are used instead of the adjectives. To make these sentences:
That's it! (Phew!)