I've been asked about overlapping transparent inks and what inks I use, so I did a little experiment...
I use primarily VanSon Rubber based ink that come in Pantone colours. Below I've mixed a TINY bit of Relex Blue and Bordeaux Red with Transparent white, adding tiny amounts until I got the colour I wanted, the overlapping ink then produces purple. The effect is watery in appearance and the choice of paper stock can also alter the appearance and colour.
What depth you want the colour if just a personal preference but don't forget it is better to add the tiniest amounts at a time, it's a lot easier to darken your colour than to suddenly find you have to add large amounts of transparent white to get it lighter :)
I just love the colours in the photo below, the starting point for a new project :)
Briar Press will give you lots of info on the subject! VanSon do not come in CYMK but come in Pantone colours so you can buy the Pantone Formula guide which include the formulas for mixing Pantone inks.
There are several relief inks on the market if your not bothered about matching Pantone colours (links available under supplies on the right) but bear in mind some of these are primarily made for fine art printmaking so if you just want to print relief art then these may be for you! These too offer a transparency extender but from what I can gather, as I have never used these inks, is that some of the colours are already transparent/semi transparent/semi opaque, so straight from the tin a blue overlapping yellow would give a green.
If anyone has any info on transparency effects with ink they would like to share, post a clickable link in the comments below, and if you want know how to post clickable links in read this from the very helpful Haptree :)