Frequently when one plays a mono-color deck, they find that they have a
painful weakness to one color in particular.  Sometimes it is possible to
"split the weakness,"  so to speak, by using other pokemon of that color that
have a different weakness, such as including psy-weak grass pokemon in a
grass deck predominantly weak to fire pokemon.  Other times, colorless
pokemon will suffice, although colorless pokemon are, as a whole, rather weak
and unefficient with energy.  However, sometimes it is possible to "splash"
your deck with pokemon of another color but colorless attacks to either offer
a resistance to your weakness or a way to counter it. 
    Why splash?  To start, it gives you a chance against decks you are
terribly weak against.  For instance, if you play a mono-electric deck
(without Zapdos), you run a painful Fighting weakness.  A Hitmonchan Haymaker
(although they are rare) will tear you apart like a turkey on Thanksgiving. 
You could include a fighting-resistant pokemon with colorless attacks (yes,
I'm talking about Scyther) to counter that weakness.  That way, if that
annoying Hitmonchan is raking in the prizes, you can bug you opponent (pun
intended) with a Scyther while they wait for a gust of wind.  Trust me, there
is nothing more frustrating than trying to counteract a resistance. 
Secondly, it allows you to deal with painful and common resistances, such as
fighting and psychic.
    So, now that you are considering splashing your deck, what pokemon do you
want to choose?  This really depends on your deck and your playing style. 
However, there are some pokemon that are better than others.  Here's a
partial list of some splashable pokemon (B=Base, J=Jungle, F=Fossil,
R=Rocket, *=only in certain circumstances):

Scyther(J)
Paras(J)/Parasect(J)
Zubat (R or F)/Golbat(F)
Charmander(B)/Chameleon(B)/Dark Charmeleon/Charizard(B)/Dark Charizard
Magnemite(R)/Dark Magneton
Dark Vaporeon/Vaporeon(J)
Dark Flareon/Flareon(J)
Jolteon(J)
Ponyta(B)/Rapidash(J)/Dark Rapidash
Machop(R)
Aerodactyl(F)
Psyduck(F or R)/Dark Golduck*
Exeggcute(J)/Exeggcutor(J)*

    Among the best of these (excluding Scyther) include the multiple
evolutions of Eevee.  Each has a colorless attack (excluding Dark Jolteon),
and evolves from a colorless pokemon (automatic psychic resistance!). 
Psyduck/Dark Golduck can be put into a Psychic deck to deal with the all too
common resistance to that type.  The same goes for Exeggcute/Exeggcutor,
athough they can go into any deck, provided that you protect Exeggcute until
you evolve to Exeggcutor.  Aerodactyl goes nicely into a deck with
lightning/fighting weakness (provided you don't use any evolutions), and will
help shut down your opponent if they play evolutions themselves.  Finally,
the charmander evolutions can be used to splash a deck, but if you choose to
use them as such, use Base Charmander and Base Charmeleon, and nothing else. 
Dark Charmeleon is incredibly weak, and stage 2's should not be used to
splash a deck, they should be the focus of the deck, if they're included at
all.  I only included Charizard because it can be used along with another
color (like lightning) and can be made to work as such, but it is just to
cumbersome to be used to cover weakness/resistance.
    Finally, one must decide how to make room for the pokemon.  The best way
to do such is to use a basic, such as Scyther, or to run the family 3-2.  You
can probably drop the energy count and put in a few D.C.E. (only if you can
use it, though), and trim down defensive trainers, such as switch and
defender, and maybe use less Energy Removal, since you'll be "removing"
plenty of energy with kills made by your "splashed" pokemon when countering
your resistance.  Finally, you may want to take out some of your auxillary
pokemon.  Playtest your deck a lot, then make any more necessary changes. 
    I'm not saying that color splashing will work for anyone at all, but it
may help those players who have a strong deck with a painful weakness to one
specific weakness.  Ultimately, once splashed, you should test out your deck
before using it in any real competitions, and make judgements accordingly. 

                                            -FossilMagikarp
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