I bought a dinnerplate dahlia bulb and I planted it in a red pot outside in our backyard but it never grew.
I have no idea what inhibits growth or what I did wrong that it never grew.
We even watered it regularly.
I don't know, since you didn't tell us where you are or what your climate is or what time of year you planted your dahlia or into what conditions (full sun, part sun, what kind of soil) etc etc. And yet you told us the pot was red!
However, if I had to guess based on commonest problems, I would guess it was overwatered/ too heavy a soil/ in self-watering or other water holding container so that it stayed too wet and the bulb rotted out.
I second that notion. Over watering.