The addition of the adjective suffix ~a to numbers produces ordinal numbers:
Unu= one Unua = first
du = two dua = second
Kvardek tri = 43 Kvardek tria = 43rd*
The usual written abbreviation appends the letter ~a (sometimes alone, sometimes with a hyphen, and sometimes raised slightly) to the Arabic numeral: 9a, 9-a, 9ª, 130a, 130-a, 130ª. The form with the hyphen is most common. They will, in the object, also take the ~n.
All of this is a preface to actually answering the question you asked. I want to be certain that people after you who read this have a clearer understanding of what I'm saying, and, hopefully, why.
"The third one is red" = La tria estas ruĝa. We do not normally need to translate the concept of "one" here, if anything that would just confuse the issue. Think of the sentence la unua unu estas …, in English the ordinal and the cardinal are different seeming (and sounding) words. Since clarity is always correct, even if ungrammatical (try NOT to let that happen) we tend to leave off such marker words.
However, this can break down when discussing multiples, but that's covered pretty well elsewhere.
As for the rest of your question: Blue, the noun would be bluo, which works for almost everything except numbers. Numbers in their "raw" form are able to be considered as nouns, so a child reading her first counting book would see "Jen la cifero unu." Not unuo! That word = "a unit", (and duo = "a couple, pair", trio = "a trio, threesome", ktp.)
Some other time we will probably discuss numbers with the ~e ending; but I feel that this is long enough.
I do hope that this helps. (kaj ne por la unua fojo.)
"Multaj lingvoj diferencas de la mia." tradukas kiel "Many languages differ from my one.", ĉu ne? Mi scias, ke la traduko pli bona estas "Many languages differ from mine." La Angla ne estas mia denaska lingvo. Eble mi ne devus diskuti pri la Anglan en forumo pri Esperanto. Pardonu!