The color in all rated games on Chess.com is chosen at random!

Though it might seem like you are getting a lot of games in a row as one color, in the end it will even out to around 50%. You can check in your stats page to see how many games of each color you've had, and verify that it is about 50% of each.

If you do want to choose your color, in order to practice a specific opening for example, you can choose your color if you play unrated games.

Hover over 'Play' and choose 'Play', then click the 'Custom' dropdown:

After you've chosen your time and type of game you want to play, untoggle the 'rated' option.

After you turn off the rated option, you will be given the choice to pick your color:

Choose your color, then click 'Play' to search for a game.

This kind of game may take longer to find a match, because not as many people want to play unrated games.

Another way to choose your color if you don't want to wait for a match, is to play against the computer.