Go to the robot cho-mechanic (i create that name lol xD) buy robots there ok ;D
To select your robot colour there colour roller besides there when you wanna bought the robot,
How to challenge? Click your friend player card and click the robot icon =)
Pss=only when he/she have robot. Heres the pic>>>>>>>>
Here the step to buy and robot i give pics>>>>>>>
I circle it ! The robot is awsome
Hope this is helping =D
~smartdetective~