I wonder the same Jeff... The most important reason I believe they're first releases is that full color version are yet to be seen.
One of the reasons the Dracula has a monochrome poster could be that Universal didn't think local audiences would fill up theaters to see a horror film, so they decided to not spend a lot on promotion.
Given it WAS a hit not only here but worldwide, that clearly changed after Dracula. Local posters for 30s Uni-horror films (Frankenstein, Bride of Frankenstein and The Raven to name a few) are full color.
The "Universal Pictures Corp. of Argentina" is the denomination the studio had locally in the early 30s. It was changed to Universal films of Argentina S.A. in the 40s.
The reason the Wolfman was made monochrome, could be also that Universal was not financially strong at the start of the 40s decade...