This is something I planned to do this year starting with yellow. This is heavily based on the researches and recommendations of Bruce McEvoy of Handprint http://www.handprint.com/HP/WCL/water.html but I have attempted to simplify for those who don't wish to get deeply immersed. If you do follow the link. Other sources of information are the books by Hilary Page http://hilarypage.com/ and Michael Wilcox http://www.schoolofcolor.com/ , although they are starting to get rather long in the tooth and many new pigments are coming onto the market not covered in these earlier works. Hilary Page does offer upgrades on her website but when I last looked it did not appear to have been altered for a while. Manufacturers websites give good information, Schminke are excellent and you can download a pdf file that gives much more extensive information than most others. http://Schminke.de/fileadmin/downloads/pdf/horadam_aquarelle_d_gb_01_08.pdf
Amongst commercial sources Dick Blick http://www.dickblick.com/categories/watercolors give detailed information on the huge range of watercolours they sell, although you have to follow several links. Another fantastic source of pigment information is http://www.artiscreation.com/ColorCharts.html Manufacturers offer information in various forms including proper chip charts, some of which can be purchased from suppliers like Jacksons. This as opposed to printed ones that are more freely available. Winsor and Newton do a very good leaflet on their artists watercolours. Holbein and Schminke also have good literature- if you can find it. The chip charts and most of the printed colour charts don't normally give pigment details but if you search manufacturers websites it is usually there somewhere. Some are easy to find others more difficult.
The starting point is that these should be single pigment paints, which are superior for mixing purposes and clarity. All are reliably lightfast although even here there can be differences between manufacturers. Bruce recommends you do your own lightfastness tests but I don't bother.
Many popular pigments like PY3 and PY97, usually called Lemon Yellow (PY3) or Primary Yellow (PY97) or similar have not been included. If you are using a yellow that doesn't appear here and you are perfectly happy with it comment by all means but remember I am not setting our `rules' that must be followed nor saying you shouldn't use this or that paint. Bruce McEvoy might but not Peter Ward. There is plenty of information about fugitive paints and most manufacturers rate them accurately but it's often in the small print! Overall there are more than 25 different yellow pigments on the market and this may be a slight underestimate. New ones, as with other colours, appear regularly. Actually if you look at the `artiscreation' link above you'll see the list of yellow pigments is enormous. However I refer primarily to those used for watercolour as listed by Handprint.
Essentially yellows come in three basic groups. Light Yellow (Lemon), Medium and Deep. The general consensus, if having only one on your palette, would be a medium yellow.
Where do we start? The obvious one is Cadmium Yellow which comes in deep/pale/light/lemon and medium hues. Two pigments are closely linked here PY35 and PY37. Incidentally P stands for pigment and Y for yellow. PY35 leans towards green and PY37 towards red. Widely available most leading manufacturers offer two or three versions. Make your choice and take your pick. The toxic nature and opaque quality of the cadmiums is something to be considered.
Next is a newish pigment PY110 tsoindolinone Yellow R, a deep yellow listed currently only by Daniel Smith and Graham. I have bought the Graham version via Lawrence of Hove but not used it yet. It is described as a warm yellow orange. Daniel Smith calls it Permanent Yellow Deep and Graham Indian Yellow (Hue).
We now come to Green Gold PY129 azomethine copper offered by W & N, Rowney, Old Holland as `Golden Green' , Graham as Azo Green and Daniel Smith as `Rich Green Gold'. This is an interesting paint that I have used for a while. It's a cool yellow that ranges in colour, depending on dilution, from yellow green to light yellow. A good one for landscape greens, particularly early in the year. I don't think PY129 can be considered as a standalone yellow but one to add if and when you expand your palette. This is one of the paints referred to as `two-tone'.
PY150 nickel azomethine yellow is another fairly recent pigment currently offered by Daniel Smith and Graham as Nickel Azo Yellow, Winsor and Newton and Schminke as Transparent Yellow. I've tried the W & N version and am not sure about it at all. So there you are Bruce! Winsor and Newton use it as one of the pigments in their Quinacridone Gold paint, a three pigment mixture.
PY153 nickel dioxide yellow. A deep yellow. This is one I use by Rowney called Indian Yellow. On my most recent courses Charles Reid had added it to his palette as Winsor and Newton New Gamboge. Daniel Smith also offers it as New Gamboge while Sennelier call it Indian Yellow. The original true Indian Yellow, something to do with cow dung (!)I believe is no longer available.
PY154 benzimidazolone yellow H3G. A neutral yellow which Winsor and Newton call Winsor Yellow. Rembrandt offer two paints Azo yellow Light and Azo Yellow Deep, while Sennelier call it Sennelier Yellow Light.
Looking for other sources I see Lukas offer Green Yellow (PY129) and Gamboge (PY153), so being well priced might be worth trying. Amongst the student quality the situation is meagre. W & N Cotman offer PY153 as Gamboge Hue so that might be worth a try and Van Gogh PY154 as Azo Yellow Light. In the USA genuine Cadmiums are part of the Cotman range BUT not in the UK (to Windsor and Newton's shame and I've told them so).
To sum up this isn't an exhaustive study by any means but a taster perhaps for further investigation if you're interested. There are many more yellow pigments but these have been singled out as outstanding. As I've suggested I'm not yet convinced about PY150 (I've had another look at it and NO I'm not) but Handprint is the place to go because he covers virtually all the currently available yellows and gives a very detailed analysis of each pigment. Some might not agree with his findings but no one else has gone into the subject in such depth. One alternative for a medium yellow is the popular PY97. I have seen different views about this pigment and have used the Maimeri version in the past. Recently I obtained some of the `Try it 'sheets from Daniel Smith.They call it Hansa Yellow Medium. I quite liked it and almost ordered a tube from Jacksons, which I may do at some future stage. Added 19.10.12: I later purchased the Daniel Smith Hansa Yellow Medium and find it excellent. Something that I should have made clear is that you find the pigment information somewhere on the tube of paint. All the major manufacturers put it on their tubes, although usually it is in very small print and almost requires a magnifying glass to read! The following website gives good general information on pigments (and much else). http://www.channeling-winslow-homer.com/
I'll conclude by again pointing out this piece is not meant to be some sort of gospel that everyone should follow. As Bruce McEvoy says different manufacturing tecniques and paint recipes means that the same pigment doesn't appear as an exact homogenous mix across the range of manufacturers. Variations exist and when choosing personal preference comes into it. In addition pigments are supplied by several firms and this is a factor in the resulting paint.
There are also other important yellows in the earth colours and they will be covered separately.