How can i set color range in 2D plot?

I am getting 2D plot by using imagesc(..,z) command but here i need white color for lower value of Z(z is a 2*2 matrix) means if my z_max 1000, then below 50 i want to get white color. How can i do this?

[SWAGATA BHUNIA]

Sorry, my Z is n*n matrix.

Answer 1

[Walter Roberson]

z_cutoff = 50;

cmap = colormap();
cmap(1,:) = [1 1 1];   %white
colormap(cmap);

zmax = max(z(:));
caxis([z_cutoff, zmax]);