Hi hiih,
I'm sorry to hear you are having issues using your Philips TV as an external display.
The following article explains some things you can try, like calibrating the display, to troubleshoot this issue.
Apple computers: Troubleshooting issues with video on internal or external displays
Update your software and firmware
Many common display issues can be resolved easily by making sure the software on your Apple computer and the firmware for the video adaptor, display, and Apple Thunderbolt cables is up to date. Check for updates for Apple products by using Software Update or visiting Apple downloads. Check with the manufacturer for any updates that may be available for displays, extenders, or adapters not made by Apple.
If you continue to have issues with brightness or color uniformity, check the Displays System Preferences pane. Choose System Preferences from the Apple () menu, then choose Displays from the View menu. From the preference pane you can adjust brightness on internal and Apple external displays on the Displays Tab, and calibrate your display from the Color tab.
Note: Third-party displays may have brightness and additional color adjustments available on the display itself.
When using a monitor via an HDMI connection, you may notice poor video quality if your display is set to television mode. Switch to monitor mode to achieve the best video quality. Check the documentation that came with your display in order to switch to monitor mode, or contact the manufactuer for assistance.
I hope this information helps ....
Have a great day!
- Judy