Why the object rendering color getting dark when I apply a normalmap to the surface shader?
I just create a sphere and apply a standard material (karma material builder) to it. Create a mtlximage and load a common normal texture,pipe it into a mtlxnormalmap keep the scale by default of value 1,then pipe into the standard surface "normal" plug,then the color getting dark , am I doing something wrong?

I'd say that's the effect of your normal map. Try a different one. Make sure your mtlxImage is not applying color correction to your normal map. Set its signature to RAW. That's not needed.

I am trying to convert some arnold material(maya) to karma mtlx material ,using the same texture in arnold there's no such effect,the color keeps exactly the same no matter I turn on or off the normal bump.It seems not the problem of some specific textures ,a lot of other materials with different textures generally have the same effect,the more bump strength applied the darker result it appears.I took a close look at the bump effect between arnold and karma, they are almost the same.The only difference is the rgb color getting darker in karma.Maybe they are working in a very different way of deeling normal bump.

I found the key information in the help doc : Shaders importing normal map images must be vector3, and not color3.
And I didnt notice that the mtlximage load the texture with type color3 by default.switch to vector3 the problem solved.