pantone 300

pantone 301

pantone 3233

pantone 5123

Etc...

Pokémon Watermelon

Pokémon Pineapple

Pokemon Red/Blue/Yellow - Gold/Silver/Crystal - Ruby/Saphire/Emerald - Diamond/Pearl/Platinum - Black/White/???

out of 14 main pokemon games only 5 are named after colours so I don't think they will run out any time soon as they have 1 more colour then 9 non colour Pokemon games before another round of colour to keep with tradition and you know gameFreak like tradition...

My guess the next round will be called Iron/Bronze/Stone after the early ages of human civilisation followed by Garnet/Topaz(or Kyanite but that sounds odd)/Jade then Obsidian/Quartz/Rhodium(or Palladium) 

they released Heart gold and Soul Silver, so you can see were they can go

But i doubt Nintendo will make over an trillion pokemon games on an ever expanding amount of colours. 

They'll start using minerals/elements again.

"Careful measurements of our visual system’s best performance have been 

made by psychophysicists (people who study human responses, like seeing 

color, to things in the world, like light). They have shown that we can see 

about 1000 levels of light-dark, 100 levels of  red-green, and 100 levels of 

yellow-blue for a single viewing condition in a laboratory. This means that 

the total number of colors we can see is about 1000 x 100 x 100 = 10,000,000 

(10 million). A computer displays about 16.8 million colors to create fullcolor pictures, really more than necessary for most situations."

Source

How many pokemon games are you expecting?

On a more serious note, even if there are millions of colors available I doubt they'll use unpopular colors.

Pokemon Old Rose

Pokemon Fuchsia

Pokemon Terra cotta (altho this sounds kinda cool)

That actually sounds interesting....

Pokemon: Neon Blue/Red/Green

Pokemon Schlong

Pokemon Melons