The color of stones is really just as important as what kind of stone it is. And when you pick a stone, as with any magick, it is important to know how this perticular stone, not the kind, but the individual stone, reacts to your own body chemistry, your feeling about the stone is always more important then the kind, or color. And make sure before buying your stones that you know that what you are thinking your buying, really is what it says it is. Alot of stores sell these magickal semi-precious stones.
Or you could just find stones in nature, preferably in a magickal spot you feel power.
Then, once you buy or find your stones, you must cleanse them. Sometimes they don't need cleansing when you find them in nature, but it's better to be absolutely sure that you don't get any negative energies mixed in with you magick. Or even if the other energies aren't negative, it's best to be sure that when you charge a stone, that's all it's charged with, it works best that way, and you can be sure that your magick is done purely.
I'v read many different methods for cleansing, so whatever has worked for you before, or what you trust will work, or whatever you'v read is best for whatever certain thing your planning to use your stone for, you should do.

Yellow Stones:
Power and energy of the mind, creativy of the mentle type, sudden changes, communication skills, heighten visualizations, travel.
Planet mecury rules yellow.
Examples: Topaz, amber, citrine, yellow jasper, sunstone (sometimes called goldstone), yellow diamond, fluorite.

Amber:
Gem of sun, fire, and spirit. It is very effective against negative magick.

Topaz:
A sun and fire stone. An all-purpose protective stone. Topaz wards against negative magick, mentel imbalance, depression, intrigue, anger, accidents and disease, while at the same time attracting money.
A form of yellow quartz is a sun and fire stone. By itself, or with other stones, it enhances psycic awareness.

Yellow Jasper:
Is an opaque form of chalcedony. Is a stone of mercruy, and air. It attracts good luck especially in connection wtih use of mental abilities.

Sunstone:
Is a sun gem. true sunstone is expensive and rare, but there is a type called oregon sunstone that is quite useful in magick. It draws money that can be accumulated through ideas. It also strengthens visualiztion of prospeity.

Yellow Diamond:
Gives off vibrations of courage, and vistory and draws good luck.

Fuorite:
With a yellow tinge to it gives helps to eliminate anger and straighten out thoughts.