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How can i change the text color of Tabs in CRM 2013 ?
There is no supported way to change the tab colours in CRM 2013 (or older versions of CRM)
I would advise you against making unsupported changes because if later you have a problem with your CRM, Microsoft won't investigate the problem because they will say it could be caused by the unsupported changes you have made.
Any method you would use to do it would involve using document, the blog below shows how to do this in CRM 2011
The following works but I just tried it and it isn't too pretty as some element backgrounds can still be seen. As Ben said, there isn't a supported way to change the tab color so be aware that this is unsupported.
document.getElementById("tab0").style.backgroundColor = '#FFF19D';