To download an icon in any color you will need a NounPro subscription (monthly or yearly). Free account holders can unlock the color tool by upgrading their account here, thenounproject.com/accounts/signup/pro/
Purchasing an icon à la carte for $1.99 will not allow you to change its color, you will need an active paid subscription.
To change an icon’s color, simply find the icon you want and select one of the preset colors options found beneath the icon. Select the dropper icon to find the perfect color or simply enter the hex value if you already have a color in mind.
Once you are happy with your color, select download and your icon will download in the color you chose.