I have made several Yellow Brick Road quilts and it doesn't matter if you choose fabrics that are all the same or drastically different. That's the fun of this pattern in particular - you can achieve totally different looks just by the fabric you choose. You could choose all purple fabrics with varying intensity, you could choose a rainbow of colors, you could select half blue and half yellow so they mix and match when cut and sewn...the possibilities are endless and this is a great pattern to experiment with.
What's most important I would say is that you like the way they all look together when you group them. Good luck! ;)