One way of changing colors throughout your artwork in Adobe Illustrator is by adjusting the color of your Global Swatches. If you're using Global Swatches as color fills (or strokes) on all of your objects, changing such a swatch will change this color throughout your artwork. You make a swatch global by checking this option in the color window that appears when double click a swatch. However, a more drastic way of changing all colors in your artwork is via the Recolor Artwork feature...