This could probably be done fairly easily with an
Arduino. You'd just need a
color sensor and the stylus could be on a
servo. Program it so when it sees a color, the servo moves to tap the screen, then back to its original position. I haven't worked with color sensors, but servos are easy to use with Arduinos. This is a good place to get help, and I'm sure others will chime in! Most of the people here are nice and helpful.
Here's something I made using a RedBoard Arduio and the servo linked above.
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