How to change the tick numbers of my code by a step of 8?

gamma=20;
%gamma=580;
P=0.1;
%P=1;
N=P.*gamma;
lamdazero=1550;
[lamdapump,lamdasignal] = meshgrid(1540:0.1:1570,1540:0.1:1570);
%beta3=1.3;
%beta4=-8*10^-4;
beta3=0.06;
beta4=-2*10^-4;
c=2*pi*3*10^8;
L=1;
%L=0.01;
A0=(1./lamdapump) -(1./lamdazero);
B0=(1./lamdapump) -(1./lamdasignal);
%Linearmismatch=beta2.*c.^2.*(B0).^2;
%beta2=(6*pi.*10^5.*beta3.*A0) +(0.5.*beta4.*A0.^2).*(6.*pi.*10^5)^2;
%Second0=(1./2).*beta2.*(c.^2).*(B0.^2);
Third0=10^-9.*beta3.*(c.^3).*A0.*(B0.^2);
Fourth0=10^-12.*beta4.*(1./2).*c.^4.*(A0.^2).*(B0.^2);
%S4=(10^-12/12).*beta4.*c.^4*(B0.^4);
Fourorder=(10^-12).*c.^4.*beta4.*(1/12).*(B0).^4;
deltabeta=Third0+Fourth0+Fourorder;
%BB=deltabeta;
%BB(deltabeta<0)=10;
colormap hsv
cm = colormap;
cm(30,:)
hold on

hold on
 for i=1:1:length(deltabeta)
    for j=1:1:length(deltabeta)
        if(deltabeta(i,j)<0 && deltabeta(i,j)>-4*N)
            test(i,j) = NaN;
        else
            test(i,j) = deltabeta(i,j);
        end
    end
 end

contourf(lamdapump,lamdasignal,test,'ShowText','off')
colorbar
 h = colorbar;
 set(get(h,'title'),'string','\Delta \beta');
%set(h,'YTick',[-1000.*N,1000*N])

xlabel('\lambda_p_u_m_p')
ylabel('\lambda_s_i_g_n_a_l')
title('Contour representing linear phase mismatch in terms of pump and signal wavelength ')

[Jan]

Do I understand correctly, that the complete code does not have a connection to the question? It simply does not matter how the values are created, when all you want to know is how the XTicks are defined?

Please explain, what "tick numbers of my code by a step of 8" exactly means.

[el-abed haidar]

thank you for the reply. So in physics the white region obtained is the signal gain region region where the contour plotting the linear phase mismatch(delta beta) in terms of pump ans signal wavelength should be between 0 and a term called nonlinearity( -4 gamma times pump power) which is -8 i want to ask if i want to also give another color not found in the color bar such as black and I want that to be representing the highest gain which will be at any wavelength for delta beta = - 4 gamma pump power how can I do that?? Thank you and looking forward to your reply