Your 18 month old should know some colors or all colors.
However all toddlers are different and learn at different paces.
Your child's ability to recognize different colors heats up at around 18 months, the same time he begins to notice similarities and differences in shape, size, and texture.
But it will be a while longer before he's able to name the colors; most children can name at least one color by age 3.
It's okay to let your 18 month old watch TV as long as they are not watching TV all day.
There's nothing wrong with allowing a kid or toddler to watch TV some as long as they are getting away from the TV for most of the day to do other things.
Also have the toddler watch good educational cartoons.
Although it's also okay to allow the toddler to watch other cartoons even if they are not educational.
I watched TV at 18 months old and as a toddler and child and I turned out just fine.
TV is not bad for toddlers as long as they are not watching the TV too much.
The toddler needs to be away from the TV for several hours and out playing when possible or playing inside the house away from the TV.
The TV should not be a babysitter for the toddler or child.