Use APKPure App
Get Image Decolorizer old version APK for Android
Colorize/Decolorize parts of an image that you want to highlight
Use this app to stand out regions of an image by coloring them.
The original image (which must be in color) is first converted to grayscale (black and white image), then you can color the desired areas using your finger.
The thickness of the brush can be regulated by zooming the image.