It appears that there is a problem with Ambiance GTK (and also Radiance GTK) when displaying a coloured symbolic icon: When using Numix(-Circle) icon theme, in Files/Nautilus the red icon "user-trash-full-symbolic.svg" is not displayed in red (RGB 220 50 47, or dc322f), but rather in a hardly visible light grey on light grey.
In contrast, with current Numix GTK from Numix Project and with current Orchis GTK from Moka Project, this icon is displayed correctly and as expected in the red colour as designed. I am attaching three screenshots for comparison.
Possibly of interest may also be this corresponding bug report against Numix icon theme: https://github.com/numixproject/numix-icon-theme/issues/240 Maybe there is a solution for this from Ambiance GTK's side.
This behaviour just as in Ubuntu 14.10 is also present in current daily Ubuntu 15.04. If I remember correctly, this behaviour started with Ubuntu 14.10, whereas in Ubuntu 14.04 LTS the icon was displayed correctly and as expected in the red colour as designed.
ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: light-themes 14.04+14.10.20141015-0ubuntu1
ProcVersionSignature: Ubuntu 3.16.0-31.41-generic 3.16.7-ckt5
Uname: Linux 3.16.0-31-generic x86_64
ApportVersion: 2.14.7-0ubuntu8.2
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Feb 28 22:18:21 2015
InstallationDate: Installed on 2014-11-17 (103 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
PackageArchitecture: all
SourcePackage: ubuntu-themes
UpgradeStatus: No upgrade log present (probably fresh install)
For comparison: screenshot with Numix GTK