To reproduce:
1. Open the attached project (attached by QA);
2. Change Editor Skin to Professional (Edit > Preferences... > General);
3. Open "TestWindow" (Window > TestWindow);
- Notice all of the text color is red;
4. Change Editor Skin to Personal;
5. Open TestWindow.

Expected result: GUI.contentColor is not ignored with Personal Editor Skin;
Actual result: GUI.contentColor is ignored with Personal Editor Skin.

Reproduced on versions: 2017.1.2p3, 2017.2.0p2 , 2017.3.0b11, 2018.1.0a6.

By design: GUI.contentColor is multiplied with the textColor specified in the current GUIStyleState (black in personal skin, light gray in pro skin). In this case the user needs to do either specify his or her own GUIStyle to use, or temporarily modify those used by the editor.