Your Walkers all behave the same(color,movement,initial position) now, at least for me. gcc's implementation of the random_device is broken on windows so if you use gcc your random number generator gets seeded with the same value for every walker. A workaround would be to use only one rng and pass that around. Your programm will do the same each time you start, but it will at least give your walkers different values.
I don't know what the others think but imo you should initialise POD-Types(char,int,float etc.) like this:
int value = 0;
instead of:
int value{ 0 };
And global variables are a big no-no.