First, health bar bouquet is locked so if you want to mess with it significantly, make a copy of it first, and select it as your bouquet.
Options-General-Indicators-Health Bar-Generic (solid), then hit Edit next to it, and it'll open it up. Type in a new name for it and hit Copy. Then go back to Options-General-Indicators-Health Bar, and select for your copied bouquet by name. Hit edit next to it again.
Then scroll down to bottom to Statusbar: Health % and click on it. Then on the left, click on the color swatch. When it pops, it doesn't matter the color you select but set Opacity to 0. I would also maybe remove the debuff coloring of the bars as that will likely throw you off so scroll up in your bouquet and remove Custom Debuff Color or whatever the flag is called.
Then go to Options-General-Indicators-Background, hit Edit next to it, scroll down to the bottom to where it says Background:Always Solid, click on it and set it to whatever you wish.
Another way to do this whole thing is to instead set the background to transparent and leave foreground as whatever you wish your background to be, then hit More next to Options-General-Indicators-Health Bar and check Invert.
Also for both methods, don't forget to go to Options-Panels-General, and for the background panel color swatches, set them both to opacity 0 (not visible) on the bottom.