in-game screenshot.

Junichi Masuda explains some of the composition challenges and decisions he made in designing music for the Pokemon series in his blog.

Discussing Red/Green:

Above softwares were developed for the game
machine called “Game Boy.”
Game Boy allow us to use total of four notes.
Coordinated by two square waves, one triangle wave
and one white noise.
Not only I composed the opening theme,
but I also composed the game music,
sound effects and Pokémon cries. All of them.
The opening music was created with the image
of the battle scenes.
“Monster” was the image of the title scene music.
I follow a guide and composed about 30 fifteen seconds
music to confirm the direction.
After this, I started total music composition.
To make the sound gravely, I added one rather heavy
white noise as if sound like marching music.
By changing the volume of white noise little by little,
it expresses a leap sound of snare drum.

Follow this link for more discussion and sound clips. Masuda discusses creating voice samples in part two of the column.