Edit:
This doesn't work! I also didn't read Vile's post close enough to see that he already tried this...
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I just did quite a research on it. The blp's that warcraft uses to determine the looks of the xp bar are in war3.mpq in this path:
UI\Feedback\ProgressBar\
There you will find these files:
human-xpbar-border2.blp
human-statbar-color.blp
human-statbar-highlight.blp
TimerBar.mdx
What does human-xpbar-border2.blp do? It determines how the border around the bar looks. It is gold-colored by default. You can change this easily.
What does human-statbar-color.blp do? No, it doesn't tell the bar to be purple (too bad). It determines which degree of purple is shown. If you make this image all white, the whole bar would be a bright purple colour. If you make this image all black, the xp bar will be all black, too. If you make it grey, the bar will be a darker tint of purple.
What does human-statbar-highlight.blp do? Te be serious, I don't have a single clue. I think it's a relic, and that it isn't really used.
And now the interesting part: TimerBar.mdx (which is quite a weird name, maybe blizzard wanted to make this bar a timerbar first?). If you convert this to mdl, you can actually alter the color! Look for these lines:
Code:
GeosetAnim {
static Color { 0.823529, 0.509804, 0.254902 },
GeosetId 1,
By altering the real values in the static Color {} tag, you can alter the color. The colors are Blue Green Red, respectively. Putting 1 here means 254 in commonly used Red Green Blue, used by several image editing programs. You can use any program that can choose colors in the 254 format to choose a color. Then divide all values by 254 and put the answers between the static Color {} tags. Remember not to use more or less decimals than you see here (6) or very weird things might happen. After you have altered the color, convert it back to mdx, and import that mdx to this path:
UI\Feedback\ProgressBar\TimerBar.mdx
Now the color of the bar should be changed. The only problem you might have now is that the bar is darkened according to the grey values in human-statbar-color.blp . This can be altered by replacing that blp with a totally white version of it. Now the bar will have exactly the color you selected in the image editing program!
Note: I haven't tested this, and I might be wrong in for example the color format, but I think this is the way it should be changed. If I did anything wrong here, please post it!