The colorant I am using came from ArtStuff.com.  I am using a black cement colorant to get grey plaster.  They come in a large variety of colors and sizes.  You can get the black in either 1 lb or 5 lb containers.  I got the 1 lb container because I didn't know how well it was going to work and if it didn't work out I didn't want to be stuck with a bunch of black powder.

My first problem was what to use as a scale.  I needed a scale that would let me make a fair amount of plaster but still have a good enough precision to get very little colorant.  I ended up using a scale I had lying around for my handgun reloading that would handle up to 50 grams and has a 0.01 gram precision.  The scale was made by Frankford Arsenal and has always been accurate for measuring powder charges for my ammo.  I'm sure you can find a comparable scale that's not for reloading online but it's what I had on hand.  For the first few experiments I used a scale that only went to the 1 gram precision.  This was when I thought that I was going to need much more plaster than I ended up needing.  Once the percent colorant goes below 1% I'm using the higher precision scale.

For my tests I picked 40 grams of plaster as my basic weight without colorant.  For the amount of colorant I use a percentage of the amount of plaster.  This way I didn't have to get exactly 40.00 grams of plaster (which would have been almost impossible with what I had to work with) and still get a good measure of how a color turned out.  In order to keep as much the same across every experiment I used the same amount of water (45% by weight of plaster) for each experiment.  Some of my results you may notice that if you do the math for plaster to colorant it won't come out exactly to the percentage I have listed.  This is because I accepted a small amount of leeway for the amount of colorant.  It all should be very close to the listed percentage.  For example the 0.2% colorant actually turns out to be 0.19925%.  The result of this error is that no two batches will be exactly the same shade of grey.  The differences should be very small and not very noticeable especially after the pieces are painted.  I did some basic tests for strength by crushing some of the plaster flakes from the sides of the cup to see if I could discern any change in the strength of the plaster.  For all tests I couldn't tell that there was any loss of strength.

For the percentages I used for my experiments I wanted a large range to give an idea of what was possible.  When I started I was thinking about 1% would probably be the lowest I would go.  After doing a test at 1% however it quickly became obvious that I would have to get much less color than I had originally though

Stats	Notes	Picture
10% 50g plaster 5g color	This was the first experiment I did when I expected that I wouldn't go below 1% colorant.  This is completely black.
2% 100g plaster 2g color	This was the second test I did to see if for some unknown reason it got much lighter with the drop from 10%.  As you can see it's still really dark.  It's not quite as black as the 10% but it's definitely a dark charcoal color.
1% 100g plaster 1g color	This is slightly lighter than the 2% but still very dark.
0.7% 40.11g plaster 0.28g color	I figured that since 1% was so dark I probably better reduce the amount of color by a fair margin for the next test.  This is more of a dark grey
0.6% 40.10g plaster 0.26g color	Still lighter in color
0.5% 40.19g plaster 0.21g color	This is very dark but might work for a building made from basalt or other similar very dark stone.
0.4% 40.19g plaster 0.16g color	This is getting closer to a good color.  The color is still a bit on the dark side for my tastes but it would work well for some applications.
0.3% 40.06g plaster 0.12g color	This is still rather dark.
0.2% 40.15g plaster 0.08g color	This is close to what I was looking for.  It's a nice grey color, not too dark and not too light.
0.1% 40.10g plaster 0.04g color	This looks good too but it's a bit too light for my tastes.  It still looks good though.

Finally a shot of all the experiments together to compare the colors.

From right to left.
Front Row: 0.1%, 0.2%, 0.3%, 0.4%
Second Row: 0.5%, 0.6%, 0.7%, 1%
Top Row: 2%, 10%

Conclusions
Wow!  A little really goes a long way with this color.  I've decided to go with the 0.2% mixture for light grey applications and 0.5% mix for dark grey applications.  At these mixes the one pound box I bought will last me 200 pounds of plaster at the 0.5% mix and 500 pounds of plaster for the 0.2% mix!  This is some very nice stuff at a reasonable price. 

Plaster Colorant Experiments Brown