Brightness sliders rarely work under 8.1 and 10, because of... well things I don't really get but they may be linked to compatibility settings applied to <Dx9 games (which, as far as I understood, work in a kind of window mode to prevent clashes with the Desktop, so they are tied to its brightness, like the real window mode. Or something.).
You could try wrappers to begin with (dgVoodoo ( http://dege.freeweb.hu
/ ), WindeD3D (you may need to alt-tab to regain control), DxWnd maybe, DXGL...) and see if they work for you.
The Age of Redemption Mod ( http://www.e-mods.net/modules/mods/about_aor.php
) should provide options to set the game to Dx9, this may solve the issue right away. It changes the gameplay though (in a good way! - I only got enough to play only half of the game with an older version, sadly, but the latest looks great!). Note that the executable may be different in this case, keep this in mind if you're using the ACT (see below).
There are ways to change/add compatibility fixes though - all of them can also solve a bunch of other issues.
A solution which works well is using the Application Compatibility Toolkit. It's a bit complicated at first, you can have a look at how it works here for example (for ACT 5.6; scroll down to post #66 - http://www.blitzbasic.com/Community/posts.php?topic=99477
). However it allows for easy testing and changes. The ddraw.dll on this page could also help.
I've made 'installers' using the Homeworld games OpenGL fix from PCGamingWiki: http://1drv.ms/1DSWrDb
. I've kept the install/uninstall files, only pointing to my fixes rather than Homeworld's, obviously, so they should work fine.
You may want to make/install these fixes yourself however. Follow the help in the post #66, or if you're using my files, after installing the ACT open the .sdb files and simply go to File=>Install (or right-click on the barrel icon). Uninstalling works this way too. You'll get a "Test application" option which will be of great help. These are the fixes I'm using:
"AllowMaximizedWindowGamma" - it solved the issue under 8.1, under 10 the second option is better.
"DXPrimaryEmulation [with parameter -DisableMaxWindowedMode]" - it's the one from the post above. May help, will also smooth <Dx9 games/16bit graphics but may also introduce flickering videos and interface. Try the third fix in that case.
"NoGDIHWacceleration" - works even better on older games (smooth framerate etc.), but may not work on not-so-old ones. This may also help if you have buggy cutscenes/intros (use it instead or with the bink.dll as indicated in the relevant thread in this forum).
If ACT 5.6 doesn't provide these fixes, search for 8.1 (or W10 according to your OS) Development Kit (ADK) and only download the ACT 6+ (the rest is alien to me ^^' ). Use the 32bit tool by the way.
Edit: changed order after some tests with Windows 10 (game locked on IntelHD, Optimus on my laptop doesn't work as intended right now, I can't try nVidia).