I'm with Jim on this one, you can add a filler to give the blank more body, or mix two colors to darken one, reverse painting helps darken a translucent blank... but only a bit. Adding a little more color slowly helps too.
Been down this path and have learned quite a bit in the past year - with much more to learn.
I started with the pearl ex powders and poured many blanks. Given the translucent nature of the colors, it is recommended that the brass tube be painted and/or reverse painting of the blank. You will find many interesting and amazing blanks with this method. Some folks vary the color they paint the tube. The varying affects are amazing.
For deeper (i interpret as opaque) colors - I recently purchased the 12 color kit from US Composites. I have poured 5-6 blanks so far and turned two. I like the affect and the richness of the color.
I also poured opaque blanks with a mix of the pearlex (green opaque/white macro pearl) ... I am heading to the shop this evening to see how they came out. If I like the first one, there will be a total of six in this shade being given as gifts to co-workers.