Are the words “negative”, “nego” and “niger”, “nox” cognates?

cognate
adjective
1.
LINGUISTICS
(of a word) having the same linguistic derivation as another; from the same original word or root (e.g., English is, German ist, Latin est, from Indo-European esti ).

2.
FORMAL
related; connected.
"cognate subjects such as physics and chemistry"
synonyms: associated, related, connected, allied, interconnected, linked, coupled, correlated; More

The human mouth is able to make about 40 unique sounds. Those sounds are represent by 26 letters in most languages. So any language is bound to have some words that sound similar even though they are not related. It is even more likely when words are borrowed from one language into another.

You need five books to study the bible, and only five.
1. A King James bible. Other versions are ok, but all the reference materials are keyed to the KJV.
2. An English dictionary.
3. An Interlinear Greek-English Translation Of The New Testament, any brand.
4. An exhaustive concordance. There are several brands, but everybody seems to use Strong's. I like Young's because it is easier to use.
5. A Hebrew lexicon, if you study the old testament.

All the words in the bible, all the root meanings, and all the translations are catalogued, even the mistakes. You don't have to guess at them.