What should I use to make a color flow like figure to look like shiny stained glass. [/i]
Color flow is one option, if you wanted to make it in advance. One drawback to the color flow is that it's opaque, so it might look more "mosaic" tile than stained glass.
When I did a stained glass cake, I used colored piping gel. I used my icing with a tip 1 or 2 for the outline, and then filled in the areas with the colored gel as I wanted it. My base was white, so the transparent colors really popped. Looked perfect
Does the piping gel have a tast or does the buttercream flavor take over; or could I add some flavor to the gel like butter flavor. I have time to make something ahead and place on the cake. This is going to be a cross cake.
Piping gel is just really sweet- no flavor. I've never tried adding any flavor extracts to it- only coloring. I don't know how to post a pic in here, but here's a link to the cross I did- http://3.bp.blogspot.com/_hbfzPKm9JcA/S7_nxIe52RI/AAAAAAAAADM/1cpOCrx_fIE/s1600/baptism+cake.jpg
You can make the cross ahead of time from colorflow or royal icing, just pipe the outlines where the piping gel will go and let dry. Once you place the cross on the cake, you can fill in with different colors of piping gel.
A couple things you should know --
The piping gel colors will look best on a white background, so make the base (flood) of your cross white. You can then pipe the 'lead' design in black or gold or whatever color you want. Make sure you let the white dry for a couple hours before piping on top with a different color.
Gel or paste coloring goes a loooong way in piping gel. You don't need much. But you'll want to make sure the colors are deep for a more dramatic effect.
You don't need much piping gel to fill in your cross design. Just a thin layer. Use a small paintbrush to spread it into the corners.