It’s easy and can often yield big results. Make it a color that really pops. The more contrast between the other page elements, the better. You want this thing to really stick out like a sore thumb. Be sure that your main call to action (CTA) or goal button stands out significantly more than all your other buttons or links on the page. If you’re limited to utilizing branded color schemes, choose the brightest color, opposite to whatever subtler color is used for the majority of other page elements. If you’re not limited in your color scheme options, I recommend using the free Adobe Kuler color picking tool to find a good complimentary color. Historically, orange is usually one of the best button colors, but you won’t know for sure until you test!