I guess I am confused where, if I guess all red, and my opponent has used one red, then I have one guess that is the correct color and is in the right position, and 4 colors that are the right color but in the wrong position. Why only one black peg then?

Say for example my opponent has a red one in the last position and I guess that color in the first position, I will get a white peg for that, but if I guess a red one in both the first and last positions, The one in the first position doesnt get anything this time?