A few paint companies provide free tools designed for previewing their paint colours on a digital photo of your project. In my experience, these can be handy (though a bit tedious) for making a choice between two or three very different options, such as: Should I paint my exterior trim light, or dark? Or: Do I want my house to be gray, or red?
For subtle colour choices, these tools will not provide enough clarity to be worth the trouble. If you've already decided you'd like to paint your house a shade of taupe, skip the digital colour overlay, and head straight for testing your paint colour with actual paint samples!
See next: Testing your paint colour