If you're planning to install shade sails at your property, you'll likely want them in colors that match either the general color scheme of the buildings on the property, or colors that match the ones used in your business' logos. This is understandable because you want people sitting under those shade sails to subconsciously recognize that you were the one who added the sails. However, when you choose the color of the sail, you have to factor in more than just your attempt at subtle marketing. The color and thickness of the sail (or the overall opacity) really influence just how comfortable sitting under those sails really is.
Heat Absorption and Reflection
You know lighter colors reflect sunlight and darker colors absorb it (and its heat), so if you really want to allow people to cool off, a lighter color would be better. This can be a lighter shade of the colors that match your logo, too. Note that you don't have to avoid all dark colors; medium shades of colors like green and blue tend not to be that bad in terms of radiating heat down into the area where people are trying to rest. If you do really want a very dark color, be sure there is good ventilation that allows for a cross breeze (take into account where buildings and things like freestanding directory signs are; you don't want those blocking a breeze), and consider adding fans under the sails.
The Problem of Glare
One color to really be wary of, if you're looking for a lighter color for the sail, is bright white, especially in a thin material. That white underside creates a lot of glare, both from light reflecting off pavement up to the underside of the sail (and then reflecting back down into your eyes) and from light actually getting through the thin material. (These reasons are why you'll see a lot of white hats with dark undersides on the brims; if you've ever worn a white hat with a white brim underside, you'll know exactly what phenomenon you want to prevent with the sail.) If you want a white sail, make it a very opaque one so sunlight doesn't shine through. If possible, look for something off-white as that will help cut down on glare.
You'll have a chance to check out sails, material thickness, color, and shape when you visit showrooms. Bring samples of the colors you want with you so that, if you decide to seek a related shade color (e.g., a lighter green than what you use in your logo), you'll be able to make a better match.
To learn more about shade sails, contact a company near you.