code/datums/greyscale/layer.dm 
| /datum/greyscale_layer/icon_state | The most basic greyscale layer; a layer which is created from a single icon_state in the given icon file |
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| /datum/greyscale_layer/color_matrix | A layer to modify the previous layer's colors with a color matrix |
| /datum/greyscale_layer/reference | A layer created by using another greyscale icon's configuration |