I want to make deep purple, dark blue & black icing. My buttercream icing always comes out lighter than I'm trying to achieve. Is there a trick to getting the colors I listed above? Special tint or just the amount I'm using?

Use lots of good quality concentrated colouring. I get on better with 'candy' (oil-based) food colouring for buttercreams like IMBC - they mix in well, whereas water-based gel colours don't take so well in a high butter-content buttercream.

Good gel color is important. Try Wilton or Americolor. Add enough to get it close to the color you're looking for, then let it sit for a little bit. (Color intensifies as it matures). I usually check it after an hour or so and add more color as needed. A neat trick I have found is to put a little of the tinted icing on a spatula and run it under some warm water. This gives me a good idea of what the color will look like after it has sat. To get black, you can do the same. However, anytime I have made black, I always started with chocolate buttercream so I didn't have to use as much color, and I wouldn't end up with that gunmetal gray color. That is just my preference though. HTH and good luck!

There are many threads on this site regarding how to obtain dark colors. I hope you do a search as there will be many, many hints besides what will be posted here.

............My buttercream icing always comes out lighter than I'm trying to achieve.........
You're just not adding enough paste/gel color I bet you are using a toothpick poked into the color. Take the foil off completely and use your small spatula to scoop out a blob and mix it in.
Dark/bold colors take lots of gel and time to develop. Always mix them at least 12 to 24 hours before wanting to use them.

For red: start by tinting your white b'cream with pink, yellow, orange or lavendar. Make it rather bright by adding a good blob or paste/gel color. Let that sit overnight to allow the color to develop. Then add whatever shade of red you want. OR if you can/want to flavor the icing, use UNsweetened KoolAid powder Yes, about 1/4 teaspoon KoolAid to 1 cup of icing. This would need to be done at least the night before you need it as it takes a long time for the KoolAid color to saturate the icing.
KoolAid can be added to any color icing to intensify color & flavor it.

For Blue - use KoolAid mixed into any blue based icing. Don't start with white but add gel color 1st to a bright shade then add the KoolAid.

Black: save all the tiny dibs and dabs of icing when you are finished w/any decorating. Put them all together into a fzr safe container and fz until you need some black. Mix it all together - you'll have an ugly color but don't worry; now just add black gel and stir well.
It's great to start with chocolate icing to get a good black. In fact many shops just use choco b'cream rather than coloring it black