I would have to say a good variety of basic colors would widen the possibilities of different hues. I say this for a couple of reasons. First, some colors like magenta, cerulean, and golden yellow will give you jewel tones. Whereas fuschia, sun yellow, and turquiose will give you brighter tones. Also, there is no true cyan in the world of MX dyes but turquoise is fairly close.
Some good basic colors to have on hand areMixing red(magenta), fuschia, golden yellow, sun yellow, strong orange, turquoise, cerulean blue, cobalt, and navy
Here are some other links that might be helpful. The first one is a brand comparison of Pure vs. Mixed
dyes. Basically, there are some mx dyes that are "one chemical" dyes vs. others dyes that are a mix of these basic one chem. dyes. Doesn't make them better than other necessarily, but it helps to keep things more simple and predictable when you are mixing your own colors.
The other is a Mixing Chart
. This chart uses colors from Jacquards line of MX dyes, but fiber-arts.com carries both lines(Jacquard and Prochem). Just in case you were curious, Dharma purchases their dye stocks from Jacquard and repackages them and also does some further color mixing. Remember that the amounts are based of the weight of goods(OWG).
Anyways...I have been kind of out of it the past few days, so if I left out any info or clarifications I apologize