OKay... rule #1 is you never want to overlay my art with your are because then when you update, the install will wipe out your art.
The bars are white because their color depends on their use... for example mana is blue, energy yellow, rage red, etc. nUI "colors" the bars on the fly based on the type of bar being displayed... health or power and what type of power.
What you need to do to replace the default HUD bars with your bars is to put your bars into the [ Interface > Addons ] directory structure where WoW can see them when it loads, then edit the HUD skin files in [
Interface > AddOns > nUI > Layouts > HUDLayouts > {HUD mode} ] -- in there you will find the
["Health"] and
["Power"] sections which define the size, location, options and artwork that are used for the HUD. It should be pretty ovbvious what you need to do once you look at that... you just need to fix the dimensions and the texture file paths to use your art instead of mine.
You could also consider doing this as a new HUD plugin and share with everyone. Have a look at the Textless HUD plugin in the 'nUI: HUD Designs' download section at
http://www.wowinterface.com/downloads/cat125.html
If you still have questions after you look at it, holler and I'll do my best to answer them.