It seems that the resulting flower will be based on the petals used, regardless of the type and quantity of petals were already planted?
I planted well over 160 white poinsettias on a big flower sprout, and then used 150 blue petals, hoping to harvest a blue poinsettia, but ended up with with a regular blue flower 
, I’m not sure if I got any blue fruit out of this either. I’m not even sure if you can bloom any other type of flower even if 150 special petals are used.
Lesson learned, only use this feature if you really want the postcard. 
Basically, if you plant more than 160 seasonal flowers (in this case, poinsettias ), those seasonal flowers will bloom.
I have experienced that 159 plants doesn’t give you the result you want.
One thing to be careful is that the number of flowers planted and the number reflected in the Big Flower are not necessarily equal. For this reason, you need to check how many flowers you have planted.
This could be due to the influence of other players, but in this case you won’t be able to check the exact number of flowers remaining unless you restart the app.