Yes they are brighter, but this is mainly about black equalizer. If you raise it to max. it will make dark spots easily visible, grass will be brighter and player model because it's blue and red stays the same. .....
As rocki already stated
If you change the brightness on your screen the player model will get brighter too and have the same colour as the barrels.
With sunman's textures that won't be the case. The player will have a darker colour than the bright barrel and you have an advantage to spot him.
With official textures you can't modify your monitor to highlight only a blue player who is standing in front of a blue barrel (because they have almost the same color) or at least to a very limited extent. With sunman's you would be able to do that pretty easily, since there is such a huge difference in color saturations. And that's just one of the examples. Sunman's (and most of the other packs) modify the way the game 'highlights' players on a map. Be it easier or harder to spot, you are still modifying something very crucial to the game, which shouldn't be possible.
Also, there is a reason why CS:GO doesn't allow external textures in official servers. First off, because mappers can rely on them (and that is by far the most important argument imo) and they knwo for sure that their map will always look consistently. The same applies to PB. Secondly, because an advantage some of the packs would offer would be highly personal view and it could stir a lot of discussion inside the community etc.
Summed up:
1) Reliability for mappers
2) No room for discussion (even though we are talking specifically about sunman's, once you allow one exception, others will want their exception as well)
3) Everyone gets the same game with exactly the same color balances and 'color interactions'