Color can change depending on a variety of factors.  (For example, the thickness of the glass and the amount of time spent at casting temperatures).  Some Bullseye glasses, mainly the 001100 series and 001400 series, are so saturated that they will appear black at certain thicknesses.
Other colors, particularly transparents that rely on cadmium, selenium, and/or sulfur, are slightly more sensitive to heatwork. If exposed to excessive heatwork, they may shift from transparent to opalescent.
The form of glass used can also affect color.  A casting made of billet will have excellent clarity, while casting with the same color in frit will result in a piece with a milky appearance.
To read about selecting glass for casting, see TipSheet 8: Basic Lost Wax Kilncasting.