Instead of

message += "The color you added was {}.".format(colors[3])

why not use

message += "The color you added was {}.".format(colors[-1])

so it will always pick up the last list element even if the number of list elements changes?

It would be even better to catch an exception if the list is empty.

try:
message += "The color you added was {}.".format(colors[-1])
except Exception as e:
print(e)
exit(1)