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Gain is the weight applied to the diffuse parameters. You may also drive this with another pattern to show things like fading or wetness (where liquid darkens a surface).
Below are examples at 0.0, 0.5, and 1.0 gain for a 50% gray material.

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Color

Color is typically where textures or patterns are connected to create color for opaque objects.
This is where a wood color texture would go, for example.

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