How to restore original screen colors on my iPad ?

You will have a wallpaper set - even if was just a flat colour.

As for what may have triggered the change, there is insufficient information to suggest possible cause. Your screenshot illustrates your App icons are as expected.

If your iPad is not behaving as expected, try a forced-restart - as this will often clear an isolated software issue:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

You can select/change your Home Screen wallpaper from iPad settings:

Settings > Wallpaper

Here you can create, customise or set different wallpapers for your Home and Lock screen.

More information can be found within the often overlooked iPad User Guide. Here is a direct link to the appropriate section of the Guide:

Change the wallpaper on iPad - Apple Support