Welcome to my world! Reds can be very difficult but not impossible.
Try this first...
Ir.Ox. Red, Warm Hue, enough Mars Yellow till it starts twords Orange, then Ir.Ox. Black to tone it to Terracotta.
If that does'nt do it for ya, then try this one... but watch out...
If you're still having a difficult time with Ir.Ox. Red, Warm Hue, try starting with Irgazine Red or even Irgazine Ruby, work in some Ultramarine Blue, yes it will start to look purplish, but then add a touch of Iron Oxide Black. The black will bring it back "down" but be careful - Ir.Ox. Black is very potent. Too much of it and you will be starting over. If you overdose on black, there is no turning back. Remember when we were kids... "Touch black - no backs." Boy, I may have just aged myself with that one, but it's certianly true with color mixing! at this point you may need to add Yellow to mellow out the "blood" color you just made. You will need to "play" with it until you reach a formula you like because all of the materials and applications techniques differ and some materials just won't accept color that well anyway.
Some Venetian plaster suppliers offer their own tints and you may want to go that route if it's available to you.