UI\Feedback\XpBar\human-bigbar-fill.blp does affect the color, but as you said it is warped. It turns out the RGB is just warped by the factors (0.545098, 0, 0.513726) (more or less). So if you have a white texture (255, 255, 255), it would become:
R: 255 * 0.545098 = 139
G: 255 * 0 = 0
B: 255 * 0.513726 = 131
Same rule applies for other colors. Sadly, because the green value is multiplied by 0, you can't have any green in it. You can get mixtures of red and blue though--or the colors themselves. You can get blue with (0, 0, 255), red with (255, 0, 0). etc. Note that you will may have to change the actual texture a bit. It currently has a gradient that fades to white (which is why you'll always end up with a magenta/pinkish hue at the top), so if you want a solid color you might need to use a solid/somewhat solid texture.