Update: The issue is not observed in Lightweight render mode
Currently text color is different in RadButton (color: #00156E) and decorated ASP Button (color: black) with RadFormDecorator for Office2007 skin. For the time being a custom CSS can be used either for the RadButton or for the decorated ASP Button:

-Custom Styles for the RadFormDecorator:

    <style type="text/css">
        .RadForm_Office2007.rfdButton a.rfdSkinnedButton,
        .RadForm_Office2007.rfdButton input.rfdDecorated[type="button"],
        .RadForm_Office2007.rfdButton input.rfdDecorated[type="reset"],
        .RadForm_Office2007.rfdButton input.rfdDecorated[type="submit"],
        .RadForm_Office2007.rfdButton .rfdSkinnedButton button, 
        .RadForm_Office2007 a.rfdSkinnedButton,
        .RadForm_Office2007 .rfdSkinnedButton button, 
        .RadForm_Office2007 .rfdSkinnedButton input.rfdDecorated,
        .RadForm_Office2007.rfdButton a.rfdSkinnedButton,
        .RadForm_Office2007.rfdButton a.rfdSkinnedButton *
        {
            color: #00156E !important;
        }
    </style>

-Custom Styles for the RadButton:

    <style type="text/css">
        .RadButton_Office2007.rbSkinnedButton,
        .RadButton_Office2007 .rbDecorated,
        .RadButton_Office2007.rbVerticalButton,
        .RadButton_Office2007.rbVerticalButton .rbDecorated,
        .RadButton_Office2007 .rbSplitRight,
        .RadButton_Office2007 .rbSplitLeft
        {
            color: black !important;
        }
    </style>