I lightly toast the bread on both sides under a grill, then butter just the edges of one side before grating cheddar onto it. If you butter the edges of the bread before you grate cheese onto it, you'll find the edges retain their colour but do not burn. The stronger the cheese, the less you need to use. I don't give colour a second thought because taste is what matters to me. If I was concerned with appearance, I would make a rarebit instead of using pure cheese.
Rarebit is fabulous for grilling. My favourite meal of all is a mackerel fillet topped with rarebit and chillies, grilled until the rarebit begins to bubble and brown, served with chips and a light salad.