Strange rose flower mystery ... (or is this common?)

 in my backyard i have a yellow rosebush, and to the right of it a pink
rosebush ... they were here when i moved in 3 years ago.

I don't know how long this phenomenon has been happening, but this year i
noticed that some -- not many, just a few, of the yellow roses were getting
pink bits on them, mostly speckles .....

I asked a few people but they weren't sure what might be happening so I'm
just wondering if anybody here might be able to shed some light on it?

Here is a photo of a couple of the yellow roses with pink spots --
http://i40.tinypic.com/2a9pt9d.jpg
You can notice at the bottom of the photo that there's a normal, perfectly
yellow unaffected rose flower -- as most of the yellow roses are, yet the
two in the foreground are 'pinkified'

 thankyou very much