HELLO (bear with me, I've never skinned any of these before)
Right, so the GMF uses hex codes for component colours. 
Here is the chart that I use.When you decompile Dome1.gmf in the Domes folder using a GMF decompiler, 
I usually use Serge's, you should be left with a notepad file that can be opened. Near the top you should find:
*MATERIAL_LIST
{
	*MATERIAL_COUNT	2
	*MATERIAL
	{
		*MATERIAL_REF_NO	0
		*MATERIAL_NAME	Material #53
		*MATERIAL_CLASS	Multi/Sub-Object
		*MATERIAL_AMBIENT	0x313131
		*MATERIAL_DIFFUSE	0x313131
		*MATERIAL_SPECULAR	0xE5E5E5
		*MATERIAL_SHINE	0.100000
		*MATERIAL_SHINESTRENGTH	0.000000
		*MATERIAL_TRANSPARENCY	0.000000
		*MATERIAL_WIRESIZE	1.000000
		*MATERIAL_SHADING	Other
		*MATERIAL_XP_FALLOFF	0.000000
		*MATERIAL_SELFILLUM	0.000000
		*MATERIAL_FALLOFF	Other
		*MATERIAL_XP_TYPE	Other
		*MATERIAL_LIST
		{
			*MATERIAL_COUNT	3
			*MATERIAL
			{
				*MATERIAL_REF_NO	0
				*MATERIAL_NAME	2 - Default
				*MATERIAL_CLASS	Standard
				*MATERIAL_AMBIENT	0x313131
				*MATERIAL_DIFFUSE	0x313131
				*MATERIAL_SPECULAR	0xE5E5E5
				*MATERIAL_SHINE	0.100000
				*MATERIAL_SHINESTRENGTH	0.000000
				*MATERIAL_TRANSPARENCY	0.000000
				*MATERIAL_WIRESIZE	1.000000
				*MATERIAL_SHADING	Blinn
				*MATERIAL_XP_FALLOFF	0.000000
				*MATERIAL_SELFILLUM	0.000000
				*MATERIAL_FALLOFF	Other
				*MATERIAL_XP_TYPE	Filter
			}
			*MATERIAL
			{
				*MATERIAL_REF_NO	1
				*MATERIAL_NAME	1 - Default
				*MATERIAL_CLASS	Standard
				*MATERIAL_AMBIENT	0x6AD0FF
				*MATERIAL_DIFFUSE	0x6AD0FF
				*MATERIAL_SPECULAR	0xE5E5E5
				*MATERIAL_SHINE	0.100000
				*MATERIAL_SHINESTRENGTH	0.000000
				*MATERIAL_TRANSPARENCY	0.700000
				*MATERIAL_WIRESIZE	1.000000
				*MATERIAL_SHADING	Blinn
				*MATERIAL_XP_FALLOFF	0.000000
				*MATERIAL_SELFILLUM	0.000000
				*MATERIAL_FALLOFF	Other
				*MATERIAL_XP_TYPE	Filter
			}
			*MATERIAL
			{
				*MATERIAL_REF_NO	2
				*MATERIAL_NAME	3 - Default
				*MATERIAL_CLASS	Standard
				*MATERIAL_AMBIENT	0x000000
				*MATERIAL_DIFFUSE	0x000000
				*MATERIAL_SPECULAR	0xE5E5E5
				*MATERIAL_SHINE	0.100000
				*MATERIAL_SHINESTRENGTH	0.000000
				*MATERIAL_TRANSPARENCY	0.000000
				*MATERIAL_WIRESIZE	1.000000
				*MATERIAL_SHADING	Blinn
				*MATERIAL_XP_FALLOFF	0.000000
				*MATERIAL_SELFILLUM	0.000000
				*MATERIAL_FALLOFF	Other
				*MATERIAL_XP_TYPE	Filter
				*TEXTURE_LIST
				{
					*TEXTURE_COUNT	1
					*TEXTURE
					{
						*MAP_NAME	Map #0
						*MAP_CLASS	Bitmap
						*BITMAP	generic_refmap2.bmp
						*MAP_AMOUNT	1.000000
						*MAP_DIFFUSE
						*MAP_TYPE	Other
						*UVW_U_OFFSET	0.000000
						*UVW_V_OFFSET	0.000000
						*UVW_U_TILING	1.000000
						*UVW_V_TILING	1.000000
						*UVW_ANGLE	0.000000
						*UVW_BLUR	1.000000
						*UVW_BLUR_OFFSET	0.000000
						*UVW_NOISE_AMT	1.000000
						*UVW_NOISE_SIZE	1.000000
						*UVW_NOISE_LEVEL	1
						*UVW_NOISE_PHASE	0.000000
						*BITMAP_FILTER	Pyramidal
						*BITMAP_MAP_CHANNEL	1
					}
				}
			}
		}
	}
	*MATERIAL
	{
		*MATERIAL_REF_NO	1
		*MATERIAL_NAME	4 - Default
		*MATERIAL_CLASS	Standard
		*MATERIAL_AMBIENT	0x969696
		*MATERIAL_DIFFUSE	0x969696
		*MATERIAL_SPECULAR	0xE5E5E5
		*MATERIAL_SHINE	0.100000
		*MATERIAL_SHINESTRENGTH	0.000000
		*MATERIAL_TRANSPARENCY	1.000000
		*MATERIAL_WIRESIZE	1.000000
		*MATERIAL_SHADING	Blinn
		*MATERIAL_XP_FALLOFF	0.000000
		*MATERIAL_SELFILLUM	0.000000
		*MATERIAL_FALLOFF	Other
		*MATERIAL_XP_TYPE	Filter
	}
}As far as I can tell, changing the bitmap doesn't do anything (not sure if it's a placeholder or something? I know it's the bitmap used for the shinywheel..). The first block in the code above controls the colour of the rivets, and the second block controls the actual colour of the disc. The same is probably true for the other GMF's.
You can change the transparency, shine etc of the component by changing the number after where it says *MATERIAL_TRANSPARENCY, or the appropriate one for the thing that you would like to change. The hex codes after *MATERIAL_AMBIENT, *MATERIAL_DIFFUSE and *MATERIAL_SPECULAR control the colours of the component. I believe the Beetlebros have a tutorial on their website about the specifics of what each one does.
If you have any questions, feel free to ask :)