Make sure you have the right paints to mix with! You can make a pink with more of a violet-red/true red, but I tried once with orange-red and had to go out and buy a second red.
I've also learned that if a color isn't bright to begin with, you'll never be able to make it brighter. It's also important to try and break the color down--Whether it's a greenish blue or a purpleish blue, if it's lighter or darker, bright or dull. To dull a color down you can add just a tiny bit of it's complimentary, but too much and you'll get a muddy grey! Acrylic tends to dry a shade darker than what you see when it's wet, so it's important to make a little swatch and let that dry to see if it matches.
Or of course, you can always go out and buy a color that looks closest to it and tweak it just a bit to match. I have lime green paint because it would've been too much of a pain for me to try and mix