Venn diagrams are the worst taught thing in all of mathematics world wide. At the very least you should end off your standard class (ending in the three overlapping circles) with an open question: "I wonder if you can make a 4 circle Venn diagram?"
7-VENN-COLOUR-BLEND ... David McCandless' post on his Information is Beautiful site (as noted below) prompted me to assign a colour to each of the 7 shapes ... ROY.G.BIV ... Red, Orange, Yellow, Green, Indigo, Violet (with 40% opacity to allow the colours to blend together). This achieves in colour what the numbers indicate in the original Black & White diagram below. Next step would be to mix the colours as pigments instead of light for a more accurate scale of blends.
Waldorf ~ 1st grade ~ Math ~ Block Plan ~ Quality of Numbers 1-12 through fairy tales, Roman Numerals 1-12, intro of all four processes (math gnomes). Work w/ concrete objects & whole body movement to explore addition and subtraction facts up to 20, division, multiplication tables of 2s, 5s, 10s, skip counting, counting to 100 forward & backward. Work on same as second block, moving into working with add & sub up to 100, introducing columnar math problems by end of year.