Hello I have this code which works but I want to color the function called deltabeta with black when delta beta is between -4N and 0 can anyone tell me how??

gamma=20;
P=0.1;
N=P.*gamma;
lamdazero=1550;
[lamdapump,lamdasignal] = meshgrid(1540:0.1:1560,1540:0.1:1560);
beta3=0.06;
beta4=-2*10^-4;
c=2*pi*3*10^8;
L=1;

A0=(1./lamdapump) -(1./lamdazero);
B0=(1./lamdapump) -(1./lamdasignal);
Third0=10^-9.*beta3.*(c.^3).*A0.*(B0.^2);
Fourth0=10^-12.*beta4.*(1./2).*c.^4.*(A0.^2).*(B0.^2);
Fourorder=(10^-12).*c.^4.*beta4.*(1/12).*(B0).^4;
deltabeta=Third0+Fourth0+Fourorder;
colormap hsv
cm = colormap;
cm(50,:)
hold on

h = colorbar;
set(get(h,'title'),'string','\Delta \beta');
xlabel('lamdapump')
ylabel('lamdasignal')