There is a frequent colloquial ending -ý in the nominative singular of neutral adjectives instead of standard -é (compare malý město x malé město). This could also be found in other cases of adjectives (vystoupil z toho velkýho bílýho auta) and in the stems of some original Czech words (svléknout x svlíknout, mléko x mlíko). "-é" variants are older.

Using Czech language in official way, it is always better to choose the standard version. In literature, language rules are often broken, so it is hard to consider Czech song Kolo kolo mlýnský as substandard or incorrect:

Kolo kolo mlýnský,
za čtyři rýnský.
Kolo se nám polámalo,
mnoho škody nadělalo.
Udělalo bác!