"The biggest reason environments should not have dark textures is that in the physical world very few materials go below a 50% grayscale value. Materials that would go beyond that would be plastics, certain rocks, and paints. It is also the opposite, [...] super bright textures would be reserved for paint, metals, and others. [...] So I tend to stay in the 50% grayscale mark or higher [...] and the reason is that I rather have a brighter texture that causes too much bounce vs a texture that is draining the life out of my scenes." -- Rogelio Olguin, senior texture artist at Naughty Dog, from "Texturing Values for Environments"
Window > Histogramor
Image > Adjustments > Levels, and ensuring the image brightness is mostly in the middle 50% of the graph, between the 25% and 75% mark on the horizontal axis.
"If you create a texture that is too dark you are limiting its ability to be bright in the game. You should remember that the texture you are creating is describing how bright that surface is when lit by a 100% bright white light. Also consider that if you paint a texture too dark or include ambient occlusion that is too dark you will inhibit the surfaces ability to show shadows and lighting. Textures with too much noise and too high of contrast will also make it difficult to read a surface's shape and lighting. [...] "Below is a practical example of how dark textures affect lighting. [The left image] is trying to fix the [dark texture] by increasing light intensity [by 500%]. You can see this doesn't help GI or the dark areas at all. The final image is using the adjusted texture with a light brightness back [to normal]. These images show that if the textures are too dark they will not result in good lighting no matter how much you try to fix it with brighter lights." -- from "Epic Games Texturing Guidelines", Unreal Engine 3 developer wiki. Emphasis added.