I get asked all of the time "How do you get your red icing so vibrant?"
There really is one main simple answer, and it comes in the word "brand."
The brand you choose to make your royal icing colors with is everything. I will refrain from mentioning previously tried brands, but trust me choosing the right high quality brand will mean everything in the shade of red you achieve (not to mention how much of it you have to use!).
My go-to brand 99.999% of the time is Americolor. If you are unable to find it locally in a kitchen store, they sell it through their own website as well as Amazon.
There is another helpful tip with achieving the most vibrant reds aside from using Americolor brand. That tip is use Americolor brand AND allow your red icing to sit overnight. Any color that you allow to sit will only become deeper, and this is very helpful in not having to use so much red coloring. I typically will add the Americolor red until I get a slightly duller version of what I want, and then I wake the next day to exactly what I want.
Make sure you completely airtight cover any icing you leave out. You can even pre-bag it so it is all ready for your outlines and floods the following day.
Hope this little simple tip helps!