Litmus test crocuses?

I was just wondering if anyone else had experienced this - I planted crocuses in all my beds last year, and now they've come up and begun to bloom I noticed something funny.

The ones in the alkaline/neutral bed have all come up yellow underneath my viburnum, and the ones in the ericaceous bed have all come up purple!

They were all the same standard mixed variety from poundland, and I can't have planted like that accidentally-on-purpose... could I?