Could Triceratops be gray in color in the real world if they were alive today since larger reptiles & land mammals are mostly dull in color?

Sure.   They may have been gray.  They may have been covered with colorful feathers but that seems unlikely for a large terrestrial animal.  We don't have definitive evidence but based on the reason you provided, it seems dull and "grey" is likely.  If you look at the ancestor of Tricerops, Archaeocerotops looks a lot like a bird.  Maybe they had genetic pressure to look colorful like a bird but that would be hazardous with predators trying to eat them. 

No one knows what color they were

Color is used to attract and to warn others 

a bright  colored animal like a poisonous frog  will be dangerous to eat so it is able to say  SEE ME  you  don't want to try me..

Some land mammals have striping or spotting for partial camouflage.

Try picturing your Triceratops striped like a tiger, or having a "jungle camo" pattern.

We don't know at this date.  Mineral replacement of ... everything would tend to remove pigmentation.

Triceratops had poor vision but a phenomenal sense of smell, and they could probably hear well. 

This points to plain gray color, like a rhino. Rhinos also have poor vision. 

Some other animals, like trex, had great eye sight but we have proof of interspecies fighting pointing to an animal that is more physical than display. So trex may have also had plain colors. 

sure, it COULD be

but

consider other large herbivores living now

not only grey ones (rhino, elephant)

but also giraffe, zebra, etc.

Also

consider their closest-known relatives

birds

which even the larger species tend to have very colorful males.

Even ostriches are striking - the black-and-white males

(though the emus are much plainer...)

could be? yes, could be pink, green, any color.

would they be? probably not, but we have no idea.