with GIMP :
Color ~> Hue-Saturation

vmg_dirt_clayey.png
	from default_dirt.png
	Hue −6
	Brightness +5
	Saturation +40

vmg_dirt_sandy.png
	from default_dirt.png
	Hue +5
	Brightness +40
	Saturation −10

vmg_dirt_silty.png
	from default_dirt.png
	Hue +8
	Brightness −10
	Saturation −30

vmg_red_clay.png
	from default_dirt.png
	Hue −5
	Brightness +10
	Saturation +100

vmg_silt.png
	from default_dirt.png
	Hue +6
	Brightness +10
	Saturation −78

vmg_fir_tree.png :
	from default_wood.png
	Hue +6
	Brightness +50
	Saturation −24

And convert to indexed color.
