ColorfulVolume is one way to get the job done. Once this free jailbreak tweak is installed, a custom color can be selected and applied to the volume HUD. Not only this, but a toggle will enable a random color to be displayed every time the volume is changed.
For those who are looking to keep a consistent color instead, RGB and alpha sliders make it possible to choose any color. The tweak also features a toggle that will apply the color settings to the ringer HUD.
A custom title can also be set, which replaces the "Ringer" text at the top of the HUD. Last but not least, Settings -> Colorful Volume includes a toggle to disable the tweak and restore the default color.ColorfulVolume is available on Cydia from the BigBoss repo for free. Jailbreaking iOS 7.1.x is now possible with Pangu. Check here for instructions on how to jailbreak iOS 7.0.x with evasi0n 7. You can also find out how to purchase and install Cydia tweaks with this guide.