Most artists and watercolour instructional books would recommend the use of single pigment paint. But are single pigment paint better than multi-pigment paint? Are multi-pigment paint more prone to mixing mud?
This is Daniel Smith's Permanent Alizarin Crimson that's made with these three pigments PR 177, PV 19, PR 149. The colour still looks very vibrant comparable to other reds in terms of vibrancy.
This is how Perm Alizarin Crimson looks when mixed with French Ultramarine (PB 29).
And this is with Phthalo Blue (GS).
The mixture created with French Ultramarine looks a bit more purplish. The Phthalo Blue mix is more muted. There's really no difference here.
How many pigments were used to create the paint is not as important as where the colour is on the colour wheel.
When we switch out Perm Alizarin Crimson for Quinacridone Rose (PV 19), and mix it with French Ultramarine, we can get lovely purples and violets.
Here's a comparison of Aussie Red Gold (PY 83, PR 101, PV 19) and New Gamboge (PY 97, PY 110). Aussie Red Gold is warmer and mixed with Scarlet Lake (PR 188), the orange mixture appears more vibrant compared to that mixed with New Gamboge.
And when we mix the three-pigment paint with another three-pigment paint, we get a warmer orange too. That mix on the left has a total of 6 pigments but the colours are still vibrant.
Here's a mix of Aussie Red Gold, Scarlet Lake and French Ultramarine. The colours were allowed to mix on the paper.
This is a mix of Aussie Red Gold, Scarlet Lake and Phthalo Blue (GS). The colours were allowed to mix on the paper.
Comparing the two mixtures above, the version with Phthalo Blue seems to be more muted. That's because Phthalo Blue mixed with a warm red will kill the vibrancy, and the addition of Aussie Red Gold won't be able to increase the vibrancy. When you mix colours, vibrancy is affect, often reduced, but never increased.
Whether you will get mud depends on how you mix the colours. If you let them mix on the paper, or use more water, the colours won't become muddy that easily. When you mix the colours completely on your palette before you apply onto the paper, you're more likely to get get mud.
Undersea Green appear more muted, less vibrant, compared to Sap Green. You can probably use Sap Green to create a mixture that looks like Undersea Green, but you definitely cannot use Undersea Green to create a Sap Green look. You cannot make a less vibrant colour more vibrant.
So regardless of how many pigments there are in the colour, you should always choose a colour that's vibrant to start with.
Colours with multiple pigments can be considered convenient colours. They save you time from having to mix them. I can mix Sap Green with other colours, but I use Sap Green often enough that it's way more convenient to have it pre-mixed.
Where the colour is on the colour wheel is more important than how many pigments were used to create that colour. It's also important to understand how the multi-pigmented colours play with othe existing colours you already have. When you know the ins and outs of your palette, you'll be able to produce consistent and predictable results.