How do you plot a circular colour map for just one variable?

I have just one variable, refractive index, that i want to plot on a circular colour map. (see attached photo for an example of what im trying to do).

Answer 1

[KSSV]

Z = randi(20,25) ;
[m,n] = size(Z) ; 
r = linspace(0,1,n) ;
th = linspace(0,2*pi,m) ;

[R,T] = meshgrid(r,th) ; 
X = R.*cos(T) ; 
Y = R.*sin(T) ;

pcolor(X,Y,Z)
axis equal
colormap(gray)