Hmmm. I don't think scansion is the issue, #.
In Spanish songs, it seems to be as easy to sing a four-syllable word on one note, as it is to sing a one-syllable word - once you get used to that. "Roja" and "colorada" seem to fit the scansion equally well. I think there's a shade of difference in meaning, or maybe it's a regional difference between Puerto Rican Spanish and the Mexican Spanish I'm familiar with here in California.
-Joe-