Ray, if you have two parameters, air pressure and frequency, you can't use a colormap and you'll have to create an RGB image instead. You can get the color like this, assuming air pressure and frequency are the row and column of your "colormap" image:
for col = 1 : columns
for row = 1 : rows
thisColor = colormap2D(frequency(row, column), pressure(row, column), :)
rgbImage(row, column, :) = thisColor;
So it has a pressure image and a frequency image. For exvery pixel, it gets the pressure and frequency and uses those as an index to get the desired color for that pixel, which it assigns to a new RGB image.