USERCOLORMAP(COLOR1,COLOR2,COLOR3,...) creates a colormap with colors specified by 1x3 vectors (COLOR1, COLOR2, COLOR3...).

When the number of input colors are three for example, the function linearly interpolates every column of a 3x3 matrix [COLOR1;COLOR2;COLOR3] to 256 values, which is to be used as an input to COLORMAP. It means that colors between those you specified change gradually.

If you put a number (>0) as the last input argument (USERCOLORMAP(...,NUM)), then the intensity scaling is respected by automatically adjusting colors, in such a way that the first color be the darkest (or lightest) and the last be the lightest (or darkest). This is useful when figures have to be printed out or photocopied in grayscale.

When using this option, n-th intensity I(n) is expressed by I(n) = I(1) + (I(256)-I(1))*((n-1)/255)^NUM. When NUM = 1, then the scaling is linear (as in colormap(gray)). 1.2 can be a good compromise between color map and intensity scale. You can check how it looks in intensity (black and white) by exporting a figure to a black/white eps format.

You can also create an m-script like
%%%%%%
function C = mycolor
C = usercolormap([0 0 0],[1 0 0],[1 1 1]);
%%%%%%
and call this colormap by 'colormap(mycolor)'.

(Example 1)
color1 = [1 0 0];
color2 = [1 1 1];
color3 = [0 0 1];
figure;
colormap(usercolormap(color1,color2,color3)),colorbar;

(Example 2)
color1 = [0 0 0];
color2 = [1 0 0];
color3 = [0.2 0.2 1];
color4 = [1 1 0];
color5 = [1 1 1];
figure;
C = usercolormap(color1,color2,color3,color4,color5,1);
colormap(C),colorbar;