Welcome to Android Central! Which phone? There are a couple of possibilities:
1. Go to Settings>Display or Settings>Accessibility and look for an option for Greyscale.
2. If the phone has Digital Wellbeing, make sure Bedtime Mode is turned off.
Welcome to Android Central! Checking for Bedtime Mode is a great suggestion. Please consider making an account with us here at Android Central, so we can better assist you; and so you can respond to your thread. Here is the link to get you started!