Grulla/Grullo is a dun "color". I think a lot of silver dapples are mis-identified as grullo/grulla due to their color even if they have no dun indicators. In your example, if the bay is a bay dun, I do think a grullo/grulla foal could result. I own several dun horses, one of whom is a grey grullo (below). I kinda like any horse just a little more if he or she has a dorsal stripe :bgrin
[SIZE=12pt]Erica's Gone and DunIT[/SIZE]
2005 31” grey-grullo stallion – National Top 10 (halter) & AMHA Honor Roll (halter)
Grandson of both Ima Boones Little Buckeroo Too (“BTU”) and Yellow Diamond Little Feather, Great Grandson of Egyptian King
pictured currently and as a foal when he was darker
THEN, here are a few of my other dun factored horses, though they are not grullo/grulla. Dun factor can be found on any horse color. I once wondered if there could be such a thing as a dun leopard appaloosa! And found out from people here that there could be, and that the dorsal "stripe" would only show up on the spots
[SIZE=12pt]Harmony Hill’s Harvest Gold[/SIZE]
1999 34” palomino dun or silver bay dun brood mare – halter and driving championships
Hopefully in foal for 2008 to “DunIT”
[SIZE=12pt]Tibb’s Sundowner (HOF)[/SIZE]
1998 34" AMHR Silver Bay Dun Gelding / Halter Hall of Fame / Multi National Champion / 20x Grand (two of those w/ me)
Grandson of Boones Little Andy -- He's my hoofed teddy bear
[SIZE=12pt]Double A Ranch's Cinco-Sundance[/SIZE]
1999 42” Bay Dun Pony Gelding
purchased at the same time we got our first mini to keep the mini company!