To reproduce: apply the Window8 theme to the entire application containing a RadButton and a RadCheckBox. Change the color of RadButton to Green and handle the RadCheckBox.ToggleStateChanged event in order to enable/disable the button. You will notice that after disabling once the button enabling it again, the ButtonFillElement.NumberOfColors property is set to 1 and gradient color becomes solid. Please refer to the attached sample project and gif file.

Workaround:

int numberOfColors = 0;

private void radCheckBox1_ToggleStateChanged(object sender, Telerik.WinControls.UI.StateChangedEventArgs args)
{
    if (args.ToggleState == Telerik.WinControls.Enumerations.ToggleState.On)
    {
        this.radButton1.Enabled = true;
        this.radButton1.ButtonElement.ButtonFillElement.NumberOfColors = numberOfColors;
    }
    else
    {
        numberOfColors = this.radButton1.ButtonElement.ButtonFillElement.NumberOfColors;
        this.radButton1.Enabled = false;
    }
}