There are as many variations on string quilts as there are quilts. Many ways to make, many ways to plan or to go with randomness. Keep in mind that random can result in ugly, so if you are going to go random go hog wild everyone in the pool. If everything is light but one or two dark pieces, either love those dark pieces because you are always going to see them, or keep them out, or add more to the mix.
Otherwise, sure, go for a color family, or a palette like fall colors or Easter eggs, or theme like nautical or whatever strikes your whim and available fabrics or desires for fabric shopping
String quilting can just be basically chain strip piecing too. The strips can be controlled, they don't have to be the off-cut ends or angles. You can sew on a foundation or not.
I would use other construction techniques but I liked the projects in the book String Quilt Revival by Baker and Sanders. I like to see how to get effects from manipulating the fabric rather than it all being random -- I've done enough of the random.
LOL, but having said that I have planned and fabric collected for a Christmas string quilt. I have the white-on-white backing fabric and will be sewing on to that. First rounds will be the same red and then strictly color controlled true red, green and white holiday fabrics in random placement. Final outcome will be sort of interlocking white background with string star shapes... I've been waiting for a project to motivate me to make it, maybe that deserves some consideration.