This is on NetLogo 7.0.4, but also on older versions, and happens in the Mimicry model in the model library (Sample Models/Biology/Evolution/Mimicry and Curricular Models/BEAGLE Evolution/Mimicry). It’s not so much a bug as it is an oddity in how the model is coded.
I found this while trying if I could modify it from Batesian mimicry (harmless species mimics unpalatable/harmful species) to Müllerian mimicry (two unpalatable species mimic each other so predators need to eat fewer animals to learn the pattern).
I noticed that on lines 57-58, monarchs are initialized as red and viceroys are blue, or colour numbers 15 (red) and 105 (blue) .
create-monarchs carrying-capacity-monarchs [ set color red ]
create-viceroys carrying-capacity-viceroys [ set color blue ]
Then on lines 161-163, for random mutations, it says:
if random-float 100 < mutation-rate
;; make a list that contains only the base-color 15-105
[ set color one-of sublist base-colors 1 10 ]
which takes the list of base colors [5 15 25 35 45 55 65 75 85 95 105 115 125 135] and then restricts it to the sublist [15 25 35 45 55 65 75 85 95], then sets the new animal’s colour to a random colour from that list.
The weird thing is that new animals, through mutation, can have the initial monarch colour (red, or 15), but new animals cannot get the initial viceroy colour (blue, or 105). The code comment also says “make a list that contains only the base-color 15-105“, but that’s not what the actual code does, the actual code makes a list containing 15 through 95. (For sublist, the first position is inclusive, the second position is exclusive.)
I think line 163 in the mimicry model should be replaced with
[ set color one-of sublist base-colors 1 11 ]