Messing around with one of these may do that.
<element id="TabsBar.InactiveTabInCaptionIconMask" bg="225,0,0,0" /> <!-- 204,184,141 -->
<element id="TabsBar.InactiveTabIconMask" bg="225,0,0,0" /> <!-- 151,225,209 -->
<element id="TabsBar.AltInactiveTabIconMask" bg="0,0,0,0" bg2="0,0,0,0" /> <!-- 151,146,209 -->
<element id="TabsBarPTop.InactiveTabIconMask" bg="0,0,0,0" /> <!-- 240,238,153 -->
<element id="TabsBar.TabInCaptionBarBlended.InactiveTabIconMask" bg="125,60,240,60" /> <!-- 60,186,109 -->
<element id="TabsBar.InactiveTabInCaptionTopIconMask" bg="255,0,0,255" /> <!-- 188,140,195 -->