sini also exists in Finnish and is possibly a Proto-Slavic loanword. It could, however, be older than that, a word originally used by the first known inhabitants of Europe (before both Indo-Europeans and Finno-Ugrics) and adopted from them by the Finnic people and the Slavic people alike. The Pashto word, شین, (šhin, "blue/green") is one potential very old relative. In Finnish, sini is used as a noun or as a prefix but the adjective always has the -nen ending.
- sininen taivas the/a blue sky (adjective)
- taivaan sini the blue of the sky (noun)
- siniristilippu the/a blue cross(ed) flag (prefix)
puna + inen ("red") works the same way, but kelta + inen ("yellow") and valko + inen ("white") have first parts that work only as prefixes but not as nouns. :)