Value describes how light or dark a colour is. This is relatively easy on a grey scale, basically starting with white and finishing with black. Between six and ten steps seems to be the norm. I have used six. However how and what value do you ascribe to different colours? The following is a value chart I made up using as guidance the `artist's value wheel' from the Handprint site http://www.handprint.com/HP/WCL/color11.html#wheel .
Apart from black the darkest value is Indanthrone Blue (PB60). Windsor & Newton, Daler Rowney and Rembrandt call this `Indanthrene Blue', Maimeri `Faience Blue, Scminke have two paints using PB60. `Delft Blue' and `Dark Blue Indigo', Holbein `Royal Blue' and Graham `Anthraquinone Blue'! Neither `White Knights nor Shin Han list this pigment, which is at the expensive end. Confusing isn't it which is one reason why manufacturers names have to be taken with a pinch of salt. Most manufacturers now put pigment information on tube paint, not usually pans, but often so small as to need a magnifying glass to read it. I have taken Bruce MacEvoys spelling as the correct one but will investigate further. On my chart while you can't read the letters (try clicking and enlarging ) indicating what each paint is the colours are clear enough. I wouldn't claim it to be 100% accurate but I think reasonably so.
Amongst the very darkest values is Diozazine Violet ( PV23) called Windsor Violet and Rowney Permanent Mauve and so on. Check the pigment information on the tube or hunt it down from the various manufacturers websites. One excellent commercial source is DIck Blick Art Supplies http://www.dickblick.com/ Dick Blick has a huge number of watercolour paints and if you click on the individual paints it will lead to pigment information. This has to be done with every single paint separately and needs a little patience.
Other very dark colours are Prussian (PB27) and Ultramarine blue (PB29), Burnt Umber (PBr7) and Quinacridone Violet (PV19). There are others so if you wish to pursue this any further, and I can understand many not wanting to do so, then get the Handprint value wheel. It can be downloaded and printed.
At the other end of the scale, apart from white, Cadmium Lemon (PY35) is the lightest value with all yellow and yellow shades coming into the lighter half of the chart and the reds, blues and greens in the darker one. This is a generalisation as there are variations so get the chart for specific details.