Hi,
When you use the colour box, in Document>Modifiers, what are you actually filling? As opposed to filling a layer.
Where exactly does the background reside?
And is there anything behind it? Can you go further behind the background?
The background color really might be better referred to as secondary color – but I think it was given this name for the convenience of Photoshop users.
Anway, at the top of your Color palette there are two boxes. The one on the right is the foreground color, and is the one that you normally paint with. The one on the left is the background color.
You can click once in the background color to make it the one that you’re painting with. The border line will move from around the other box to be around the background color box to show that it is now the primary color. Like with Photoshop, this is a handy way to toggle between two colors.
Also, in the upper left corner of each of the color boxes is a little triangle. If you click on it, you can turn it from gray to blue and back to gray again. When blue, it means that ZBrush will use that color for painting – if the current tool will allow it, that is.
For example, with the Simple Brush you can have both foreground color and background color active. Try it once with blue for the background color and green for the foreground color, with both boxes’ triangles being turned on (blue). When you paint on the canvas, you should get a stroke that is blue at the low, outside points, blending into green at the high, center points.
I’m reasonably certain that The Saint used this technique to create his way cool nebulous background in Cheetor.
Thanks Aurick
but you missed the question. I was not referring to the colour palette background but the one you fill when you hit the black rectangle in the Document palette. I have been trying to work it out and I can now see that the document is actually the clipping plane.
So my next question is what is the Use of the clipping plane what reason would you want a coloured clipping plane. 
>what reason would you want a coloured clipping plane?
to start painting a sky for example or to make transparent gifs.
Sarum, I still don’t get it. If you want a sky then you would just paint it on a layer wouldn’t you?.
With the document fill color, it fills the background of all your layers and any new layers you create with a plain, flat, materialess color. If you fill an individual layer, it fills it with the currently selected color and material or if you have a texture selected, it fills the layer with the currently selected texture and material.
The main difference is with the document color, is it’s useful if you just want a plain old, flat color for the background.
Hi Kruzr,
I’ve been creating a super low poly head all day so my brain buds are bursting at the moment. But your information looks good to go. I’ll have to check it out in more detail later. Thank.
Also thanks to Shinzaho for answering as well.
>. If you want a sky then you would just paint it on a layer wouldn’t you?.
If i just use a layer i wont be able to use the eraser brush properly 
Hi Kruzr,
I finally got around to checking out your post after filing it away in my scrapbook and forgetting about it.
Excellent info you have discovered. I’ll bet not many people realise the difference. It can be useful not having to deal with the clipping plane. 