The 81A is often called a warming filter, one that shifts the color
temperature slightly. Here's a decent page that should help you
understand filters:

http://www.bythom.com/filters.htm

After you read through it, if you have further questions, fire away.

I don't use a 81a and I use a colored layer( /blend/ color) to
simulate one. Though i cannot compare therefore; my gut impression is
that you can do an 81a in photoshop. Take that and run with it - it's
simple to do. I suppose my main considertion would be as to
whether or not there is an "information" derogation in the phot0 -
related to application process of photoshop (vis.. a layer blend, a
filter, or an adjustment layer).

Then too...there is the camera interaction with it's white balance,
and color sensitivity and encoding. It aint film. Dunnoh.

rgds
Ken

IMO, you can do so many things with post processing, many photogs get and
use filters to protect the lens and nothing more, unless it's a filter for a
special application like a polarizer, infrared, star, etc.