I have a bride that requested royal purple frosting on her cupcake mountain and cakes. and I've run into a bit of an issue achieving that color. Any advice would be appreciated.
Intense colors like that need time to 'develop'. Color it at least the night before and by morning it should darken several shades. You don't indicate what problem you are having so it's hard to help you any further.
the issue is that I can't seem to get it dark enough.
you can add some black and you could brush petal dust over it afterwards too -- it it's crusting icing or fondant
Depending on which brand of color you are adding it may be just a matter of using more. Some of those need as much as a teaspoon or two in a cup of white icing. Start with a different base - like a deep pink or blue or even lavender before adding the purple. Isn't royal purple a deep, deep lavender blue?
Royal purple is pretty much LSU purple. I'm using Americolor. Would adding that much color paste make it taste bad?
You might try a spray color on top of the purple icing you already have (I've seen cans of purple spray - but I don't know how dark they are). Or, if the client is OK with it - sprinkle a little dark purple edible glitter - it would add dark purple highlights.
What kind of frosting are you using? American buttercream, Swiss or italian merengue buttercream, etc...
It makes a difference in the advice.
American buttercream, made with shortening (not my first choice. but its April, in South Texas, and there is no A/C).
Americolor is pretty concentrated but purples are tough. Particularly the royal. With AMBC letting the color develop is key. Overnight is best. You will want to get about a shade lighter than your true color before letting it develop, which will mean a lot of coloring; however, have never had trouble with bad taste using americolor. K8memphis and Kakeladi also had some key advice.
Best of luck!
As for it tasting bad - it might but amp up your flavoring - at least 2 Tablespoon to each batch using 2#s of sugar. A good combination is 1 part vanilla; 1/2 part butter flavor and 1/4 part almond (I've known some people who prefer lemon instead of almond).