Some questions about materials

They’re all set to Linear, except Color and Normal maps. I don’t know exactly what each one means. I mean, why all grayscale textures are Linear Color and not Grayscale? Also, when should I check sRGB and when should I not?

I feel like I’m doing this wrong, but it’s indeed looking better. The first one is with the new method and the other is with specular = 0 for comparison. Is that what you meant?