TAKE A PICTURE
Let's say you want to pick an accent color. Take a picture that shows the current color. Point to the color in the photo to select it. The app matches it to the closest color in the system. From there you can look at four coordinating colors. The colors seem to be two pairs, each pair containing one lighter color and one darker color. (See the bottom photos.) Select one of the colors. Done.
Another approach, using the iPad app, after you have 'pulled' a color from your photo; you can select it to see its details. This will include a list of any 'collections' that include it. From there select 'Explore Color' and select one of the collections to see other colors that are part of the collection. The collection will contain several colors that "go with" your starting color, but not all will be great choices. Some discernment is required.
By the way, there is an iPad app as well as an iPhone app. They are very similar but the features are more numerous on the iPad version.
Since the App is free, I suggest getting it and playing around with it to see if it is a 'keeper'.