The color for detail objects are set in the detail settings using the 'Healthy' and 'Dry' colors and can't be changed in the detail object's material.
A different coloring method will be used depending on the shader that your detail object's material uses.
Hue, Saturation, Value (HSV)
This is the default coloring method for all the Open World Nature shaders. It uses Hue/Saturation/Value color correction to change the color of your detail objects. Nature Renderer will check the original color of your detail objects' texture and set the HSV values in the shader to most accurately match the 'Healthy' and 'Dry' colors.
Color or Tint
This is Unity's default coloring method and is used if the detail object's shader does not contain Hue/Saturation/Value properties. With this coloring method your detail object's texture will be multiplied with the 'Healthy' and 'Dry' colors.