mulch fade

Hello,

I'm completely baffled by this one. I was hoping someone might be able to offer some insight.

I bought a yard of black mulch last Sunday. I managed to spread about half of it Sunday morning, but the rest had to wait until Monday. The photo I attached shows them both side-by-side....same batch, just one day difference in when the mulch was spread. Sunday's (right) looks great, and Monday's (left) has lost all of its color. The mulch was sitting in the bed of my truck, under a tarp (and in the shade).

Any ideas on what could have happened? I'm very new to landscaping, so I could have done something wrong, but it seems very strange to me. Should I have left the mulch uncovered?

Thanks,

Rob