How do I create a shader that uses the built-in SpriteRenderer color?

I looked at the default sprite shader and it has:

         _Color ("Tint", Color) = (1,1,1,1)

However, when I create a custom shader with this property, the SpriteRenderer does not set it to the color in the inspector. I can still set the color directly via material.SetColor, but I wanted to use the built-in SpriteRenderer methods.