In the last two posts, I told you that zinc yellow, cobalt blue, and fuchsia produced the most vibrant colors of any combination of 3 Premo primaries. In the last post we saw that my statement held true for the blues and yellows. Zinc yellow and cobalt blue produced the brightest greens.
Now let’s look at the purples. We have three blues and two reds to work with.
In the image below, the blues are mixed with increasing amounts of fuchsia. On my screen (and I hope on yours) the top two strips are more vibrant than the bottom one. The purples mixed from the turquoise blue and fuchsia (bottom strip) are lovely; however, they are a bit faded compared to the other two blends.
The purples mixed from cobalt blue and ultramarine aren’t as markedly different from one another as the greens were. The reason is that the red undertones in ultramarine aren’t as noticeable when ultramarine is mixed with fuchsia. If you look closely though, especially at the dark end of the range (in the box below), you’ll see that the purples mixed from cobalt blue look a bit brighter and “more pure,” thus more vibrant, than those mixed from ultramarine. 
The more blue there is in the mixture, the more we can see the dulling effect of the red undertones of ultramarine. If the undertones were fuchsia, there would be no dulling, but (as you will see below) red mixed with blue is very different from fuchsia mixed with blue.
I was so disappointed in kindergarten when I mixed red and blue poster paint to make purple and got such drab results. The yellow undertones in red are responsible for this outcome. When ever you add a third primary (in this case yellow, from the undertones) to two others (“pure” red + “pure” blue in this case) the mixture becomes a bit duller. This is very useful to know when we want to “tone down” (desaturate) a color that is too bright (just add a bit of the third primary).
These pendants make use of desaturated purples and greens. (Click for a larger view.)
There are many situations in which these toned-down purples are needed (think of red grapes, eggplants, purple plums and autumn leaves). I will write about them in a future post. Meanwhile, look around you at the purples you find in nature; few of them are as bright as those mixed from cobalt blue and fuchsia.
Finally, note that we could alter the cobalt blue purples to look very much like the ultramarine purples by adding a teeny bit of red, but the reverse doesn’t hold. There is no way we could alter ultramarine to make the bright blue-blue violets we see towards the left end of the cobalt blue strip.
