Even if you are able to do detective work to find out the brand/color of mortar that was used, mixing it up at this advanced time from the original can mean that the patch won't match. Chisel a small hunk out and visit a brick showroom that also will have mortar samples. A whole bag will be the minimum purchase, unfortunately. Also buy a bonding agent. Chisel out as much loose stuff as you can, apply the bonding agent and mix up the mortar. Apply and wait until it's mostly hard and then tool. If the color doesn't quite match a week or two after it's cured, you can very carefully use some artists acrylics thinned out to make it match better if it's not in a direct heat location.

Hollysprings, thank you for the suggestion. I apologize for the late acknowledgement; sometimes family can intervene in our plans.