I have noticed that brown bread or wholewheat bread chars more quickly than white bread when you toast it. This is a question of albedo.
White bread reflects more radiation than brown bread, which is why it appears whiter.
Because the darker breads absorb more radiation, in the form of heat from the toaster or grill, they heat up more quickly and therefore burn sooner. A way to check this would be to bake batches of white bread with food colouring added to the dough and toast the results. Another factor is also involved. This produces flavour as well as colour in the form of browning of the bread. I always knew, deep down inside, that on the European mainland we do things differently.