Pigment numbers are handy, and certainly have their place. They do NOT give a true comparison of colours. For example most manufacturers use the same PY35 pigment for Cadmium Yellow Lt., Medium, and Dark. Pigment size and density can make a substantial difference in the visual appearance of the actual paint.
Regardless of which paint we use, our paintings are unlikely to look like Kathleen's work, so don't get too strung out over details. We see light differently, and we feel scenes differently. Colour is relative, not absolute. If your relative values are correct, the actual colour is much less important to the viewer.
The value of using Naples Yellow and Cold Gray is convenience. Mixing them yourself is possible, but consistency suffers. When using any colour as part of a limited palette, consistency of value and colour matter.
Over the years I have accumulated far too many acrylic colours. Convenient in the studio perhaps, but I have learned the true potential of few of these colors. My current project is to cull tubes that are seldom used, any with fugitive (less than Lightfast I) pigments, and as many mixtures as possible. Single pigment colours are best for acrylics, especially if one uses any earth colours. My plein air palette will be down to six tubes plus white and a custom chromatic black. The studio will have some optional yellows, reds, and earth colours added to that mix, a far cry from the assortment filling my storage unit.
The final cut will contain colours that will provide me the range of colour that I want to work with and let me get the maximum from every tube I buy. The actual pigments available for paint manufacturers is not evenly distributed around the spectrum; we have to compromise. If you want exact, actual colours, take a photograph instead. The colours will still be inaccurate, but people accept them as correct. I have seen authors suggest less than 20 percent of people see the same colour when looking at any given object. Paint what you see and stop trying to paint what you think you see. Your paintings will actually look much more believable.
