The hardest part about getting brown is getting a shit with good divergence. Takes a lot of patience with bowel control and the like. If you do it right, your ass can lase at a nice, dark brown color.
Ideally you want a wavelength that is equal parts 405, 650, and 589.
So, we use wavelength summation to get
1/405 + 1/650 + 1/589 = .0057053902
1/.0057053902 = 175.2728498745nm
Obviously, our answer is now in the UV spectrum. But what we must consider is that it is not actually on the same plane as typical light, so what we really have is a set of coordinates for a xyz graph.
x= 400-700
y= 400-700
z= 400-700
The x is 405nm
The y is 650nm
The z is 589nm.
That is to say...it is difficult to put an exact nm measurment on it. In fact, it is simplest to simply take a shit and view it for yourself. But remember, squeeze your asscheeks or you will likely get an undesirable divergence. My record is 1.3mRad if I remember correctly.
Measurements above 2.5mRad will likely cause plumbing damage, so wear safety glasses always!