I'm trying to figure out why your friend said that and what they meant. The only thing I can think of is that more muted colors make your brown/grey look richer and more bronze. However, I'm loving the rust color it takes on in your sample square. It's got a contemporary rustic feel. If you stick to that same level of brightness (meaning color saturation not light vs dark) for the other circles and stars I think it would look very bright. I don't really think any of your fabric could be garishly bright because of the marbling anyways.
The big question is what are you putting around your circles