Custom Skinned planet?

If there is such a way, it is through <SurfaceImageOverride> in PlanetArtDefs. However, I suspect that is an overlay for either the colony screen or invasions.

What do you mean by custom skins? Most planet art is defined by certain dds images, then colors applied over the top if required.

Clouds, alraeady colored, you can recolor existing dds files to change the colors:
        <AtmosphereTextureFile>Planet_Atmosphere_Clouds_Generic_Yellow_L.dds</AtmosphereTextureFile>

The Ground cover, what tiles you can build on (Overidden by  <CustomContinent>Earth0</CustomContinent>) :
        <BaseLayerTextureFile>Planet_Base_Rock_Generic_02_Merged.dds</BaseLayerTextureFile>

The initial Colors Water(I think)
        <BaseLayerGradientFile>Planet_Base_Brown_Gradient_01.png</BaseLayerGradientFile>

The initial Colors Land (I think)
        <AltLatGradientTextureFile>Planet_Generic_Continent_Gradient_Yellow_01.png</AltLatGradientTextureFile>

Type of Polar caps, Lava or Ice.
        <PolarCapTextureFile>Planet_Cap_Generic_Lava_01.dds</PolarCapTextureFile>

You can then tweak colors by changing:-

    <BaseLayerDiffuseColor>
      <Red>255</Red>
      <Green>255</Green>
      <Blue>102</Blue>
      <Alpha>255</Alpha>
    </BaseLayerDiffuseColor>

    <EmissiveColor>
      <Red>255</Red>
      <Green>255</Green>
      <Blue>153</Blue>
      <Alpha>255</Alpha>
    </EmissiveColor>

    <PolarCapTintColor>
      <Red>255</Red>
      <Green>204</Green>
      <Blue>204</Blue>
      <Alpha>255</Alpha>
    </PolarCapTintColor>

    <AtmosphereColor>
      <Red>255</Red>
      <Green>255</Green>
      <Blue>102</Blue>
      <Alpha>255</Alpha>
    </AtmosphereColor>

If I am not mistaken, I think Syrkres wants to wrap a .png around the planet so that the surface of the planet on the main screen is the .png image, as in GC2. As you point out, in GC3 a planet surface is a bump map with a gradient laid over it. This makes it exquisitely complicated to give a planet the appearance you want. I assume Stardock has some (in-house?) software that makes this a whole lot easier, because it sure isn't easy otherwise. If possible, what Syrkres proposes would circumvent this complication and allow the software-challenged a chance to mod planet art., so the question certainly has some merit.

    <CustomContinent>Earth0</CustomContinent>

Will overide the settings. And apply a custom look, like the current Home system planets. Theres a command like the cheat command you have to use to get the game to load custom continents, but I forgot what it is.

Based on the art of Earth:

<BaseLayerTextureFile> is a bumpmap using the alpha channel for height. It gives the basic distribution of 'altitude' (color) on the planet

<BaseLayerGradientFile> is the color of the lowlands or ocean. I am assuming that this fills the part of the texture map below the 'waterline' (zero height on the bumpmap).

<AltLatGradientTextureFile> is the color of the continent itself, presumably parts above the 'waterline'

Both of the gradient files are basically on Cartesian coordinates. The horizontal (x) axis is height, read low to high (low heights on the left, high on the right). The vertical (y) axis appears to be latitude, with low latitudes on the bottom, and high latitudes on top.

As far as I can figure, the bump map is 'filled' by the gradients, with each 'pixel' in the rendering having its color determined by the alpha value (its 'height') and its coordinates on the appropriate gradient. So, the top right corner of Planet_Continent_Earth_Gradient_01.png is white, which means that pretty much any height will be white at the poles (but this will be partially covered by the icecap anyway).

I thought this had more to do with where the actual hexes appear on the colony screen. Does it also relate to the configuration of continents on the planet? How would that jibe with the bumpmap/ gradient system?

Im 90% sure it overides it. You could be right thou as it only refers to the "tiles". I have not played with it much.

Yep your right. It still uses the bumpmaps to set the colors of the continents but settign a custom continent will make sure the ground cover looks how you want it:-

  <ContinentDef>
    <InternalName>Earth0</InternalName>
    <ExcludeFromRandomGen>1</ExcludeFromRandomGen>
    <EstimatedClass>12</EstimatedClass>
    <TileCount>8</TileCount>
    <TextureTile>Planet_Continent_Earth_Merged.dds</TextureTile>
  </ContinentDef>

I'm sorry, I'm a little dense here. What do you mean by ground cover?

The Land, Continents ect.

Thanks for the info, I will see if get my head wrapped around this and mod a planet.

I thought I read somewhere that custom height maps were broken? 

Thats what you need the command for. To tell the game to reload assests or some such.