Wow, thank you very much for all out answer!
Now I see - Colored Alpha is deeper than we thought
.
We can use 8 bit per channel RGB alpha. Should be ok now. Will use whole RGB now.
"Chromatic Alpha" - good to know!
Thing that Photoshop does not support in it's masks. Using multiple layers compositions to replace it.
One more thing we use for our purpose with Alpha. Maybe someone will find it usefull:
We need smooth edge "atmo-space". It is ok in 32 bit per channel, but not smooth enough in 8 bit.
So we found solution:
Swap your Photoshop to Linear workspace (necessarily!) (It works correctly if you doing it in Linear workspace only!)
32 bit per channel EXR Alpha (with Gamma 1.0 default) -> apply Gamma 2.2 (in Linear space) (Merge layers)
Convert to 8 bit per channel.
Save to RGB Alpha (TGA for example).
Then in our shaders reverse operation, 8 bit per channel Gamma 2.2 -> Gamma 1.0 (float), and then per channel operations in linear space.
We loose some details in opaque parts but have much smoother transparent parts.
About difference in Alpha with different light:
I don't think small difference in alpha couse any problem.
Thanks again!