After the German orthography reform you're supposed to write every noun with a capital letter. It's always been that way, but there were some inconsistencies before (there still are some), so after this reform it was decided that adjectives made nouns were written with a capital letter.
Now, as to why this
Neues is a noun. There are some words in German that turn an adjective into a noun. Among them is this little
etwas. Here are some more:
etwas, nichts, alles, jemand, usw.
You have to be careful, though, since these words have other functions and can stand alone in a sentence.
There are other ways of turning an adjective into a noun, of course; e.g.:
Das ist eben das Blöde!
Blöde is a noun in this case because of the