You cannot mix a surface shader ('closure') with an RGBA shader such as flat, otherwise, the result of the surface shader is lost. You need to use two shaders of the same type such as standard_surface.

 A video that shows how to use the mix_shader can be found here.


Mix Weight

Shader 1

Shader 2

Add Transparency

Shading network used in above example (left). Brick wall shader mixing concrete and brick shaders (right).

Workflow Example

Animated facing_ratio shader -> mix_weight. Snow shader -> shader1, Mountain shader -> shader2.