from someone who has been to concerts where both LinLin's and Masaki's colors were worn and represented in many places, their colors are really close but not exactly the same. Masaki's color was a little brighter, and a little bluer.
LinLin's was like this:
lllllMasaki's was like this:
lllllLinLin's was not the color people generally think of when they hear the word
teal anyway, i think fans took to using it mostly because using "aqua" or "turquoise" might get people picturing a color closer to Reina's light blue, so it was less confusing (or maybe it was just more memorable because it's not used very often). with Masaki at least we have the benefit of the name being officially called Mint according to UF and Morning Musume themselves.
and because Masaki's color can look blue when not next to an actual blue, here is Reina's for comparison..
llllllllll