Are colored eggs bad candidates for use in pots?

[lotis146]

To provide a Calcium supplement. I'm new to this process so I'm not the one to answer this question in depth, in fact how much egg shells should one add and so they be broken up to small pieces of finally ground?

[wintergirl]

I grind mine up by putting the dried shells in a bowl and use a thick drinking glass as the grinder like a "mortar and pestle" type of thing. Then I sprinkle a little bit on the top of the media. When that top layer is gone I sprinkle a little more. It seems to be working since I have not had any black spots since I started doing it.