Sorry, I have no pictures, but you can look at the sprites on another website if you want to.
During the time of Pokemon Yellow, the Pokemon designs were nearly finalized, but colors were not entirely complete until the GBA came out.
Some noticeable color changes:
Mewtwo and Mew were yellow with an orange glow (which looks pretty cool to me.)
Butterfree was bright blue with a black torso.
Pikachu was white with yellow highlighted areas.
Oddish was green, while its evolutions were red.
The Bellsprout line was a lime color.
Dragonite was brown.
Slowbro's Shellder shell is pink.
Many Pokemon were unnecessarily turned dull green: Ditto, the Rattata line, the Magnemite line, Tauros, the Geodude line, Eevee, Porygon, Aerodactyl, the Machop line, Starmie, the Cubone line, the Rhyhorn line, Dratini, and Onix.
Omanyte and Omastar's shells were blue.
Although not a color change, The 95% of Gastly's body that is made of gas is wispy, making it look like a purple flaming orb.