Find the gauge in the file and look for this line:
Code:
<DrawLinesFill>false</DrawLinesFill>
Change it to:
Code:
<DrawLinesFill>true</DrawLinesFill>
Near the same area there will be a section like this:
Code:
<LinesFillTint>
<R>0</R>
<G>0</G>
<B>0</B>
</LinesFillTint>
That area controls what color the fill line is.