At this time, Annie Sloan Chalk Paint does not offer a dark purple (or bright pink), pre-mixed, so knowing how to mix your own is the only way to add it to your color palette.
I used the same process that was used for the red-yellow color options, a 1/2 cup measurement for equal parts. The middle column shows what happens when Pure White is added to each result. These are similar to the purples that are available pre-mixed from Chalk Paint.
If you are trying to match exactly any of the Chalk Paint colors, you may be disappointed. Annie Sloan's formulas for paint colors are more complex than merely mixing 2 of them. But, if you want to add a range of purples to your palette, this is an easy and inexpensive way to do it.
Here are some ideas found on Pinerest to use custom mixed purples.