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ColorFunction and/or ColorOutput - mkdk

I would like to color certain regions of a density plot.  I have been
working with ColorFunction unsuccessfully so far.  I was wondering if
someone might be able to help me without taking too much of their time.

I would also like to clean up all the errors I am generating, by either
defining my function in a different way OR turning off a few of the
error codes.  I realize there are undefined regions in the plot.

Here it is.   It was simple enough I didn't even define functions.

cond = {alpha -> Pi/6, lambda -> 2.15, Ei -> Ep - Es}

R = lambda Ei Sqrt[ 1 - (Ep/Ei) Sin[alpha]^2 ]

DensityPlot[R /. cond, {Ep, 0, 6}, {Es,0,4.5}, Mesh->False,
PlotRange->{0,1}, PlotPoints->100 ]

What I would like to do is the following:

R<0 be black

R=0  some bright color.

0<R<1 be shaded where R~0 is dark and R~1 is light.

R>1    be white

R undefined   be some other color.

I'm sure this is possible.  And I don't need a full solution.  But maybe
an example using an IF statement with ColorFunction.

I have come pretty close so far, but the shading from dark to light is
screwed up and doesn't seem linear with R from 0 to 1.   What algorithm
does GrayLevel use to normalize it with the function from 0 to 1?  Does
anybody know?  That might help.


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