Border collies have so much variety, especially if they're from working lines, that it's totally possible she's purebred! Smooth coated border collies with her colors definitely exist, and I could personally see her being all BC, especially if she has a lot of typical BC behavior. You can try a DNA test like Embark (the most reliable out there imo) if you're really curious, but even the best DNA tests aren't 100% accurate yet so it's still going to be an estimate, albeit a more precise one than just guessing based on her looks.