Why is my screen changing from light to dark all of a sudden?

Quite what you are seeing is unclear. Lets try to rule out a possible screen of other hardware issue…

Take a screenshot of your screen when the problem is seen:

Take a screenshot on your iPad - Apple Support

By whatever means are available, perhaps email, send the screenshot to another computer where it can be viewed. If the image, when reviewed on another computer, looks the same as you can see on your iPad - then you have a software issue.

If instead the image, when reviewed on the other computer, looks as it should - then you have a hardware problem that will need assessment and repair.

Have you changed your Display & Brightness settings - selecting Automatic?

Settings > Display & Brightness > [Appearance]

My screen changes colour if i type in search i cant use it when this happens im clueless to why it happens