Some please help! Im making a cookie monster cake but i accidentally bought navy color gel, thinking i had tried royal blue and now the icing is a yucky navy colour! Does anyone know how to make it a bright dark blue without royal blue coloring? Trying to avoid going to the store again! 

Sky blue with a tiny touch of black (like microscopic tiny) should do it I imagine. Just try it on a small amount of icing first.

Are you starting with a pure white icing base or an off yellow one?

I don't think there is any saving the navy coloured icing for this project unfortunately.

he was always just straight royal blue for moi

you can send my husband to the store for you -- he's always real good about that heheheheh

It will depend on what your base color was to start with.   Maybe, just maybe, you can try a small bit to see if adding white icing will help any.  Otherwise, just freeze your navy for another project in the future.