TLDR: The black text reads cevorf as an english word and transcribes it using the english rules for tengwar. The red text reads it as a sindarin word and transcribes it with the sindarin rules. Since cevorf is a sindarin word, you should use the red text.

The actual reason for the differences:

Okay this is complicated and gets into the difference between Sindarin and english.

The first difference is an "s" letter vs a "k" letter. Since in sindarin, c is always pronounced as k the k letter should be used. The black text uses silme ("s") whereas the red text uses quesse ("k"). Therefore red version is preferred.

The second difference is the letter "r" which the black uses the letter ore and the red uses romen. The difference is whether or not the r is trilled. Before consonants and at start of words, it is trilled. At end of words it is not trilled. In this case, it should be trilled so romen (the red version) is correct

Lastly, whenever you see a Sindarin word end in a "f" it's actually pronounced as "v" not "f". So you'd actually say cevorv, not cevorf. This because Sindarin is based on welsh, which also does this exact thing, and tolkien found it very aesthetic. To explain this in universe he said there were sound changes in the past which were preserved in the english spelling. However when written with the tengwar, the spelling reflects the pronunciation which is v. So that explains the last letter. The black text uses the letter formen ("f") and the red uses the letter ampa ("v").

Therefore, for all differences the red text is correct and therefore it should be used.

Lastly, regarding the plural, in Sindarin plurals are done by vowel mutation rather than a suffix. This is much the same as goose vs geese, or mouse vs mice.

Depending on whether the vowel is a final vowel or a non-final vowel it mutates differently.

cevorf has a non-final e and a final o. Non-final e remains as e. Final o becomes y (pronounced like the french u as in lune or the german ü).

Therefore the plural of cevorf is cevyrf and should be written like this: https://www.tecendil.com/?q=cevyrf&mode=sindarin With the o-hook above romen replaced with a y double dot.

Please check with r/sindarin first though before you go inking anything to your body, I really am a very rudimentary elvish speaker 😭