I have a map of different colors, I need all z values from greater than 0 to be red, all zero values to be white, and all below zero values to be blue

color specifications

imagesc(z)
cmap = [1 0 0 ; 0 1 0 ; 0 0 1] ;
colorbar
colormap(cmap)

I applied the code above, but it just seperated the ranges equaly.

Answer 1

[Walter Roberson]

Zz = (z>0) - (z<0);
imagessc(Zz)
colormap(cmap)
caxis([-1,1])

The caxis is there for protection in case z does not have a mix of negative and positive values.

If there can be nan in z then

Zz(isnan(z)) = nan;

[Megan Mirkhanian]

Is there a way to label the colors as high, none and low?

[Walter Roberson]

h = colorbar;
h.Ticks = [-1 0 1];
h.TickLabels = {'low', 'none', 'high'};

Answer 2

[Shivaraj Durairaj]

You can specify the clims argument to the imagesc function.

The values in z that are less than or equal to cmin (-1) map to the first color in the colormap. Values greater than or equal to cmax (1) map to the last color in the colormap. Values between cmin and cmax linearly map to the colormap.

For example,

z = randi([-10,10],[4,4]);
imagesc(z,[-1,1]) % clims = [cmin cmax]
cmap = [0 0 1;1 1 1;1 0 0]; %[blue;white;red]
colormap(cmap)
colorbar

Note: The above implementation works well when elements of z are integers. In case of non-integers, you may consider to limit the values of the elements to -1,0,1 after initializing z.

z(z<0) = -1;
z(z>0) = 1;
z(z==0) = 0;