To use emoji, simply tap the  icon to open the emoji selection menu. To switch back to your keyboard, tap the  icon.

Some emoji are available in different skin colors. If you want to select a different colored emoji, tap and hold the emoji you wish to use and select the color you want.

How to make all Whatsapp emoji Brown?

When you select a different colored emoji, it will become your default emoji.  So, if you wish to make all of your emoji brown colour, then once you will have to select all the emojis one by one of brown colour.  Then the next when you will tap or use any emoji then its colour will be brown.

Its fantastic.  You can make all of your emojis of your favourite colour.