I see lots of answers about using different colours to match different areas of the quilt. But I think you are asking differently - substituting a similar colour in an all-over quilting design when you run out? I have done that - I had a bunch of half-full bobbins in various shades of brown for my longarm. I had a quilt with a really busy back, so I just used up all those odds and ends in the bobbin on that quilt. Looked fine. I would only do it if I was sure the changes would not show, or I could place them to look intentional, however. If the backing was plain, I would not have switched colours on that quilt.