Render an SCNGeometry - the output color in fragment shader modifiers not the final color to display

Question

I'm using SceneKit and ARKit on iOS and I have a face geometry and want to change it's color.

but I found the _output.color is not the final color to display. For Example, a fragment shader modifier is like this:

_output.color.rgba = float4(0.5, 0.5, 0.5, 1.0);

the face color to display should be RGB(127, 127, 127), but the fact is RGB(188, 188, 188).

I don't know why this happened, Can anybody tell me the reason and introduce the workflow of the geometry shaders. Thanks