Good Afternoon danuja001,
We see that you're concerned with a change in color on the screen of your iPhone when swapping between apps, and we'd like to see if we can help.
Are you only noticing this while using Dark Mode, or does this occur while Dark Mode is disabled?
We'd recommend checking out the following resource to see if True Tone may be related to what you're seeing. Try disabling this or enabling this setting to see if you notice a change.
Adjust the brightness and color temperature on your iPhone, iPad, or iPod touch
If adjusting the setting doesn't change what you're experiencing, we'd recommend that you Contact Apple Support so they can look into this with you in more detail.
Have a great day!