To reproduce:
1. Drag and drop RadCheckBox and RadRadioButton on the form
2. Set the Checked/IsChecked property to true. 
this.radCheckBox2.Checked = true;
this.radRadioButton2.IsChecked = true;
3. Apply the Office2013Light theme  
4. On button click set the Enabled property to false. 
The ForeColor is different when disable checked item and unchecked item(see attached image - RadCheckBox_Disable_Checked.png). 

Workaround: 
Apply custom StateManager of RadCheckBox/RadRadioButton. 
In the attachments can be found sample demo with all necessary modifications.