You need first to extract the two textures files form SkyColors_Atl.dat file using the Timetraveller SH3 Unpacker tool. Then open the reflex_atlantic.tga with a Image Editor (Photoshop or GIMP). I use GIMP, it's free. Then select only the night sea texture and change the brightness parameters and save. Do the same with the the Sky_atl1.tga file, but here you need to mess with alpha channel. Save them, repack and all is ok!
If you prefer I can do it for you, ok?
Rubini.