Screen changing colour and lines

When I unlock my iphone 5c the screen starts to dim and lines appear all over the screen and it goes a purple colour. When I press the lock button and it shows the time etc the colour goes back to normal. Then when I slide to unlock again lines appear and goes a funny colour. Each time i press the lock button it goes back to normal then whilst the screen is turned on it dims and goes the funny colour. Hope this made sense. Any ideas of problem? Screen replacement most likely?