Just quickly and from memory (so if I make a mess of it sorry)
EBC is the colour of a 10% solution i.e. 100 g in 1 Litre, the way it's measured is fairly complex but that's the nuts of it.
So for each ingredient you need its EBC mass and the volume it's going to be diluted into (the volume including losses from the kettle onwards) so call it what's in the kettle at the end of the boil).
Might be easier to work through an example, let's choose a pretty standard beer like a pale ale at 1.050 and say you have 27 L in the kettle at the end of boil, assuming your brewhouse efficiency is 75%.Just as an example.
Malt / Kg / Colour
MaltEurop Australia Ale 5.700 Kg 4.1 EBC
Crystal Pale (UK) 0.250 Kg 48.8 EBC
Chocolate Malt (UK) 0.050 Kg 500 EBC
So the colour contribution for each malt would be = (KG malt X Colour (EBC) X 10)/Volume, added together for each malt
Colour = {(5.7*4.1*10) + (0.25*48.8*10) + (0.05*500*10)}/27 = 22.43 EBC
I wish to God it was that simple, in the real world malt usually comes with a range value (i.e. Pale crystal 40-60 EBC) so unless you have the COA (certificate of analysis) there is going to be an inbuilt margin of error. Unfortunately that 22.43 is the pre-boil colour; if you look at the COA attached you will see a pre and post boil colour for a base malt (they never give boiled wort colour for specialty malt I suspect because it doesn't change much already having completed its Millard reactions).
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The boil wort colour test is a 2 hour boil, then the wort is toped up to the original volume and the colour is measured, the people in malt labs I have talked to say that the colour development is fairly linear.
So if a malt gave 4 EBC at the start and 6 EBC after 2 hours and you were only boiling for 1 hour, you would expect to gain 1 EBC.
Redo the calculation with the colour of the base malt as 5.1 rather than 4.1 EBC gives you an expected wort colour of 24.54 EBC.
Just to make it more fun, Graham Wheeler suggested once that if you're only getting 80% sugar extraction wouldn't you also be getting 80% of the colour.
With the help of some friends who have access to some smoking hot colour measuring equipment we are trying to pin down exactly what's going on.
The formula above will get you close, if you want to play around with the boiled wort colour you should get closer still, in the meantime there are so many variables that beer colour is as much an art as a science.
MHB
Edit
Should have said that you need to look up the colour of any kits and/or extract you add to a brew, then workout what the colour contribution is from any grain you steep.
Some of the manufacturers are very good at giving the specks for their products (notably Muntons) who give the colour and bitterness of every kit. A lot of them give the colour at can concentration and usually in Brix and you have to work out what the colour will be when diluted.
M
