Need help ID'ing portabella look-a-like, thanks

Hay fun guys & gals, Yesterday my partner found these pic'ed portabella look a likes next to a pile of horse manure we got for our gardens from a horsey friend. The picture was taken under incandescent lighting in our kitchen, but outside when found in daylight and under today's daylight a yellowish sheen could be seen on the stem. In the pic it can only be seen on the mushroom's ring. The bulk of my mushroom picking was on the coast and of portabellas i've only picked them from the isles of a produce store. However since moving inland I've learned, for instance, the difference between chantrelles from the coastal region and those same here in the Rockies (continental divide). So a portabella may be a bit different here than there, but a yellowing may be a bad sign, I dunno for sure. I'll retake the picture if needed, but here it is as is cuz short on time right now.  Much thanks for any input, at least somewhat informed please , OgreNick

PS: Oh, the yellowing is showing to me on the pic today. Must've had a bout of yellowblindness yesterday

Stropharia rugosoannulata, garden giant

Wow!!! The garden giant, the mushroom of my dreams -- finally! I've been trying to grow these for a while, inoculating in more than one place....2, actually, but never in the front yard. I bet it came in with my friend's horse poo. She's an avid gardener. I do hope you're right. Feelin dreamy right now. G'night and Much Thanks for answering. Looks right as I checked here too: http://www.mushroomexpert.com/stropharia_rugosoannulata.html

OgreNick

PS: I guess I'll just throw some of these into my compost tea bin, as it'd be good to spread them that far and wide. Is my guess a good way to propagate this king of garden kings? And agaric is as far as I got with the online taxonomic key. I think i used mushroom expert's. Is there a better mushroom taxonomic key out there in the WWW?  Thanks2