Colours mature at a lower temperature on ceramic pieces than on porcelain or hard china.
If after firing, you discover some black specks in your glaze, it may be caused by the use of a dirty brush, greenware, bisque or other tools you have used. Your kiln, if dirty can cause this as well.
Poor colour in coloured glazes can be caused by over-firing. This is especially true of red glazes which are usually fired at a lower temperature.