How to make part of a plot in color and part in black and white?

Hi, I have two plots and and one is in color (plot 1 - it is plotted in the jet colormap) and one is in black & white (plot 2 - it is plotted in the gray colormap). I am trying to overlay Plot 1 onto Plot 2 such that the pixels of Plot 1 that are above a certain threshold in intensity replace the corresponding pixels in plot 2, and are seen in the jet colormap as they are originally, but the remaining regions of plot 2 (the pixels corresponding to those which are below the threshold in plot 1) are from the original grayscale plot of Plot 2. How would I go about doing this?

I know how to do the composition of a data file that is the plot 1/plot 2 combination, but don't know how to display it so all original plot 1 pixels are in jet colormap and all original plot 2 pixels are in the gray colormap.

If this has already been answered, please just paste the link as an answer; I couldn't find it.

Thank you in advance for trying!

[Walter Roberson]

Are these line plots, are are they scatter plots, are are they image() / imagesc() / imshow(), or are they patch() or surf() ?

[Ak K]

They are plots of an (X,Y) data matrix, called using the plot() command.

[Walter Roberson]

colormap has no effect on plot() . plot() uses axes ColorOrder and ColorOrderIndex properties to determine which RGB color to use for a line if the plot() call does not include a color specification.