There is yet another alternative. The old ENB mod, which was presented here a long time ago (see my signature "got bloom").
This does not modify the shaders, it works in a different way, but it will work ONLY if your FSX option "DirecX10 Preview" is disabled (so, it works only in DX9 mode).
There ware a very nice package made by the people from REX, containing both the ENB module and some custom settings, but I cannot find it on their website anymore.
I have uploaded it here: http://sapdaube.free.fr/fsx/REX_ENB_SUNLIGHT.zip
It should contain 3 or 4 files to be placed directly in the FSX main folder (where the FSX.exe is).
This package will tranform the colors of your sim to make them a bit "warmer" so to say.
It will still be a bit dark though.
You might be able to alter this by decreasing the value "Darkening amount day" in the ENBSERIES.INI file (with notepad). Each modification requires a sim restart.
Might be worth a try