On my start menu, the tiled part, all my icons have the same blue background, except Firefox and Thunderbird which are butt ugly gray. Chrome was also gray but I managed to change it by renaming chrome.VisualElementsManifest.xml to chrome.VisualElementsManifest.xml.bk and reselecting the icon. Cannot find similar files for the 2 Mozilla programs. Just changing icon does nothing (it does change the icon but retains gray background).
Also tried a utility from Windows Club designed for this purpose but it can't touch these 2 icons.