Dear Zuzana,

Colour charts are a difficult problem. Each one finally becomes out-of-print and thus obsolete. I have started using HSV (Hue, Saturation, Value) colours as these are an intuitive transformation of the RGB system used on computers. This system is universal and numerically based. I made up a chart for my own use and offered it at http://website.nbm-mnb.ca/mycologywebpages/EssaysOnFungi/Collecting_mushrooms_for_scientific_study/Colours.html. Just click on the sample plate to download the whole thing. It is designed for mushrooms and is less discriminating in the green shades used for Penicillium. I use it on my cell phone, which gives very good colour reproduction compared to desktop monitors.

David

Martin, I agree that colour measurements are pretty subjective. However, living organisms themselves are variable in colour and I find some kind of approximation to be better than none, especially when working with dried collections that have lost all their colour. Photographs are also helpful for colour, but also unreliable. I just turned to HSV because it is readily availble and probably as good as what my camera sees. Your work in archaeology probably deals with materials that are less variable in colour than fungi, thus requiring a more precise system.

I've just been reading about archaeology and ancient DNA. Wow, isn't that earthshaking! We need to deal with whole-genome DNA in fungi, but so far that seems to be out of our realm.

David

I worked with two of them but never liked the results. Perhaps they are too precise and don't give a good average.

Dear Martin and David, 
many thanks for your replies.
Yes, naming colours is subjective, that's why I was thinking about a colour chart as a possibility how to make it a bit more objective.
Well, maybe providing a colourful photographs together with the description can make the same service...? 
I didn't try any mobile applications, thanks for suggesting (although I am not sure how well it can work with so little objects like Octospora). Best, Zuzana