Changing the color of stained glass is difficult, but possible with a bit of patience and preparation. The only way to do it and still have true stained glass is to have the piece cut out and replaced with a piece of glass in the new color. That requires a professional and can be costly. The color in stained glass is either the result of certain ingredients in the glass itself, or a special paint that gets fired onto the glass at 1100 degrees, neither of which is a practical home project. There is, however, a "cheat" for home crafters—glass paint.
Using glass paint will not change the color of the piece, but rather relies on color mixing to produce the desired effect. For instance, if the piece is red and you want it to be purple, you would use blue paint. For this reason, this technique is limited—it won't change red into green, for instance, but a number of color changes are possible.
Cover work area with newspaper. The pigments found in glass paint can stain most surfaces.
Lay glass bottom-up on work surface, and wipe with a damp rag to remove dust and dirt.
Clean the glass with cotton balls and alcohol to remove oils and residue. Once the glass is clean, do not touch it with your fingers to avoid re-contaminating it with oils.
Pour glass paint into the squeeze bottle and screw the cap on very tightly to prevent leaks.
Flood the glass within the lead solder outline with paint by applying gentle pressure to the squeeze bottle. Do not touch the tip of the bottle to the glass to avoid forming air bubbles. Cover the glass area with paint until you have a uniform coat no more than two millimeters thick. If excess paint gets on the lead solder, wipe immediately with a damp rag.
Allow to dry undisturbed. Apply clear topcoat using the same method if extra gloss is desired.
Mask the lead solder outline around the piece in question with painter's tape. Wipe clean with a damp rag, then clean with cotton balls dipped in alcohol. Allow to dry.
Pour glass paint onto a small plate. Mix appropriate color, if desired.
Apply a thin coat of paint to the glass with a natural-bristle brush. Keep the coat as uniform as possible, and hold the brush at an oblique angle to avoid leaving brush marks. Allow to dry.
Apply a second thin coat of paint, and allow to dry. Repeat until desired color is achieved.
Apply a clear topcoat using the same method if a higher gloss is desired.