Ok I just went and tested this. Here is what I did. And I will attach a tga file with my fill pattern as well.
I created a fill pattern with the colors I wanted the bar to be. Then I defined the tint for the gauge to be:
R=255
G=255
B=255
This way the fill pattern comes out to be white. So that when the gauge is displayed on the screen it comes up when full being the colors that I made the fill pattern. Here is the tga file I used and I will show you the code that I used to fill it in the player window.
Here is the code from the player window. I renamed the fill pattern to use the pattern from this new file instead of the original fill pattern.
<FillTint>
<R>255</R>
<G>255</G>
<B>255</B>
</FillTint>
<DrawLinesFill>false</DrawLinesFill>
<EQType>1</EQType>
<GaugeDrawTemplate>
<Background>A_GaugeBackground</Background>
<Fill>Test_GaugeFill</Fill>
<Lines>A_GaugeLines</Lines>
<EndCapLeft>A_GaugeEndCapLeft</EndCapLeft>
<EndCapRight>A_GaugeEndCapRight</EndCapRight>
</GaugeDrawTemplate>
As you can see I used the same standard pieces for the rest of the gauge and just changed the fill pattern.
Here is the fill pattern. I am not attaching a Screen shot because in this case the screen shot came out to look just like a regular gauge only with the colors from this fill pattern.
Of course I am not much of an artist so this is not a great job but there is no reason why someone else here that is an artist couldn't make something really great.