Red glass and outline of blue glass are slightly more opaque.

All colors of stained glass use identical transparency values:

• 163 for the border

• 102 for the main part

• 155 for the two streaks

Except for red glass. Red glass uses these values:

• 223 for the border

• 128 for the main part

• 172 for the two streaks

Actually, blue glass is slightly different too. It uses:

• 180 for the border

• 102 for the main part (unchanged)

• 141 for the two streaks

Those are the only two exceptions, the other 14 colors are uniform and identical. This actually causes MCPE-91361 in bedrock edition. Since bedrock does not support partially transparent particles, it only displays particles for parts of the texture that are greater than 50% opaque.

It may be intended to use different opacity values, since the eye may perceive them differently for different colors, but that doesn't seem to be the case now since all other glass textures are identical.