I think that one color decks are the best and people should play them more often.  The reason why I think this is because you always draw the right energy, so your Pokémon will be powered up fast.  The only two color deck I use is my haymaker, which does well with more than one colors because the Pokémon only require one energy to attack.  I think that mono grass and water decks are especially best because they have two weaknesses and no one has resistance to them.  Here are some tips I would make for making a one color deck of each color.
 
GRASS:  Doesn't have any major problems, just two minor ones.  Some pokémon have weakness to fire, and fire has powerful attacks, which means that your PKMN will die in one hit.  Here is a very good solution I thought of myself: Dragonair.  It's colorless, so it goes into any deck, and It's got a built in Energy removal, which is a big weakness for fire.  Include Pluspowers because grass has status changing attacks which are low damaging attacks(but don't get me wrong, Grass's attacks are the bomb!!)
 
WATER:  Well, It's obvious, Water has no beatdown basics, which is a big problem against aerodactyl decks.  Don't worry, now that Lapras and Articuno are out, water has two good basics.  And you might want to include scythers because they are colorless and are a great basic.
 
FIRE:  Fire has many discarding attacks.  Just add in lots of energy retrievals.  Fire PKMN almost always have weakness to fire, so add in lots of colorless PKMN, like chancey, to stall and have a different weakness to battle with.
 
PSYCHIC:  Very weak to resistance and confusion.  Simple.  Add some Colorless attackers and add full heals or switches.
 
FIGHTING:  One big problem:resistance.  No problem, include some colorless attackers, like fearow or Persian.
 
ELECTRIC:  Two major problems.  Ground types have both resistance and strength to them, they hurt themselves.  For the first problem, include colorless bird types with resistance to fighting, like Dodrio or Fearow.  For problem 2, Include defenders.  I would suggest using Electabuzz, Base set Raichu, Fossil Zapdos and Fossil Magneton, the best electric PKMN, In your deck.
 
Always include around 24-26 energies in your 1 color deck and watch out for the PKMN you are weak to.
 
                                                                                -Tyler C. the Grass King
 
Any questions?  Disagree?  E-Mail me at tymy1223@aol.com or tymy1223@directweb.com