It's time to take your phone in for repairs, the screen is going... Well to be technical, your pixels.
Now sometimes when you change a page, the screen hasn't fully realised, and the pixels need to change. However, now you say it's it happens often and has now.gotten worse, I'd say your screen needs replacing.
IF less than 2 years old, and still under guarantee, I'd take it back, with receipt and request a new handset.
All mobiles in the UK from new come with a 2 year guarantee. CEX, unlike most second hand seller, gives a one year warranty.... Most usually only a few months.
Each little dot on a mobile phone is made up of 3 tiny LEDs and a combination of those LEDs shining in various combo.of the 3 colours, is what gives your.screen it's colour and pictures. A dark them is recommended, as the black LED uses less power, so the darker colours use less battery.... Also better for the screen life.