To change the footer colours using the Twenty Seventeen theme in WordPress; go to:

Admin > Appearance > Customize > Additional CSS

Add in the following code:

.site-footer { background: #2000a1; }

.site-footer {
    color: #ffffff;
}

.site-footer a {
     color: #ffffff;
}

.site-footer a:hover {
     color: #ffffff;
}

The #ffffff is white, and #2000a1 is a blue colour. The output should look like this: