One of the final stages of creating the internal lightmap texture for both block and skylight is mixing the colour with 4% of 75% grey, to brighten the blacks as to not leave the player in total darkness.

However, this affects the whites of the lightmap, dimming them from 255 to around 252-253 (and possibly even dimmer since it seems to be done twice??).

A minor issue, but an especially annoying one once you notice it (especially on torches and glowstone).

A fix that will leave the blacks at about the same level as they would with mixing with the grey would be to do 

1.0 - (1.0 - out_colour) * 0.97

Instead of the current

mix(out_colour, vec3(.75), 0.04)

as the colour gets clamped between 0.0-1.0 anyway.