Colour identification in philately can be a difficult task, but it can make a huge difference when identifying your stamp and its value. Oscar Young, a professional philatelist, uses the rare Prussian Blue as an example on the colour key.
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Stanley Gibbons: How To Use A Colour Key – The Rare Prussian Blue
Another really good video. good choice. The number 1 stamp on my wants list!!!!!!!!!!!
Best wishes Michael