I've given a custom mesh and a vanilla mesh the same vanilla texture (so they would match with eachother). They match, but the texture on the custom mesh is darker than the vanilla mesh. I've tried messing with the Material sliders but it still won't get the same effect as the vanilla mesh? What am I doing wrong?

Does your custom mesh has vertex colors? ("Has Vertex Colors" set to yes on the NiTriStripsData blocks)

i forget what its called i think its defuse color in the shading part in the nif in nifskope

A mesh has vertex colors if the "Has Vertex Colors" property on a NiTriStripsData block is set to yes (this is something you can check in NifSkope).

If your mesh has vertex colors then those could make it darker than the vanilla mesh. If your mesh doesn't have vertex colors the vanilla mesh may have vertex colors which make it lighter. You can turn vertex colors on and off by double clicking the value of "Has Vertex Colors" in NifSkope. With vertex colors off they should have the same color.