It depends on which direction you want the darkness to go. If you look best in a darker blue-red tone, mix it with blue eyeliner. If you are going for a red-brown, mix it with brown eyeliner.
If you have trouble doing this, you can hold the eyeliner under the hot water to soften it, or if you are really hard core and nobody is watching, take a Bic lighter to it and it will fall in molten drops onto your mixing surface, and be cool enough to touch and apply to your lips while still nearly liquid, at which instant you dab bits of it onto your pink-lipsticked lips until the desired shade is obtained.
Or you could just go to CVS and get you a nice $2 one in the color you want.