Previous to unity-5.8, compiz-0.9.7.2, as you reached the lower values the panel would become truly transparent.
Now it remains colored until 0.000
Unfortunately 0.000 still produces this bug, previously was able to achieve a 'transparent' panel & circumvent the bug with a setting of 0.0100, no more
Bug 924592
ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: unity 5.8.0-0ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-20.32-generic-pae 3.2.12
Uname: Linux 3.2.0-20-generic-pae i686
NonfreeKernelModules: nvidia
ApportVersion: 1.95-0ubuntu1
Architecture: i386
CompizPlugins: [core,composite,opengl,compiztoolbox,decor,place,resize,session,mousepoll,dbus,regex,commands,move,text,gnomecompat,vpswitch,snap,animation,workarounds,fade,cube,expo,rotate,scale,ezoom,unityshell,scaleaddon]
Date: Fri Mar 23 17:45:20 2012
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Beta i386 (20120301)
ProcEnviron:
TERM=xterm
PATH=(custom, user)
LANG=en_US.UTF-8
SHELL=/bin/bash
SourcePackage: unity
UpgradeStatus: No upgrade log present (probably fresh install)
I can also confirm this bug with Unity 5.12 in Ubuntu 12.04 at values down to 0.01 using Ubuntu Tweak. However, I still get a completely transparent (uncolored) menu bar at a setting of 0.00.