This is the contents of the corresponding entry in the 1968 edition of Oxford Latin Dictionary:
uenetus³ ~a, ~um, a. [app. VENETVSI]
Sea-blue (as the colour of one of the circus
factions); (masc. or neut. as sb.) the Blue faction.
The meaning of the abbreviation app. is apparently and the brackets [ ] enclose etymological remarks.
In the entry VenetusI we find:
VenetusI ~a ~um, a. Of the Veneti (on the Adriatic) or their country.
Another interesting entry is this one:
Venetiānus ~ī, m. A supporter of the Blue faction in the circus.
So, uenetus was the colour of one of the circus factions and the etymology of this word seems to be the adjective Venetus, which refers to the Veneti people or their country.