Garfield's alarm clock is an alarm clock that often "speaks" by means of thought bubbles. Garfield usually destroys it by smashing it. Its color has changed over time (it is often changin between red,green and yellow). The alarm clock is a recurring character in Garfield comic strips. It is hated just like the Spiders in comic strips (which Garfield also smashes). Except Garfield smashes the alarm clocks because they wake him up.
The fact its color changes might be because it's never the same clock. Sometimes it appears to have emotions. Once, after waking Garfield up, he slept in Garfield's bed.