Choosing your color when issuing a challenge

I am in a chess club and we have tournaments.  For the tournament matches, we are obviously told which color to play.  Keep in mind that these tournaments are not official site tournaments.  So, we have to issue a challenge to our opponent so that the game can happen.  When I issue a challenge for the 45/45 game, I don't see any option to choose which color I will be.  

I did find some information about typing in a command 

/challenge player123 90 30 rated white

but, I don't know where I am supposed to type this.  This site seems to be all graphical, not text based.

You can't choose your color on rated games. You can do it on unrated games however.

You can choose colors for a rated game for a specific opponent, but not using an app or when issuing an open challenge.

The challenge command noted above (and others) can be typed in the Message line on either of the tabs in live with the bubble icon.

As you noted, the graphical interface is pretty standard on the site and you can issue your challenge that way. Just click on the Play tab in live chess, then choose Play a Friend or Custom Challenge depending on whether or not you are friends with your opponent. Then select your opponent, time control, and color (and choose Rated). The command method is helpful if you find it difficult to find your assigned opponent in order to challenge them.