Customizing mask or background in images

I want to set a mask (background) for values less than 'Int_bound' in all the figures/subfigures (code shown below). This mask could be of any color but should not appear in the colomap or should not match with a color in the colormap. Any leads will be appreciated. Thank you.

maskfctr=Int_bound;
figure('units','normalized','outerposition',[0 0.1 .8 .9],'Name','Mappings selection - Tick off relevant figures and press continue');
s1=subplot(2,2,1);imagesc(NormIntm); axis image;
set(gca,'XTick',[],'YTick',[]);caxis([0 1]); 
title('Total intensity [a.u.]'); c1=colorbar;
s2=subplot(2,2,2);
imagesc(FWm,[round(min(FWthres)),round(max(FWthres))]);axis image;
set(gca,'XTick',[],'YTick',[]);title('FWHM [nm]');c2=colorbar;
s3=subplot(2,2,3);
imagesc(Barym,[round(min(Barythres)),round(max(Barythres))]);axis image;
set(gca,'XTick',[],'YTick',[]);title('Barycenter [nm]');c3=colorbar;
s4=subplot(2,2,4); 
imagesc(Peakm,[round(min(Peakthres)),round(max(Peakthres))]);axis image;
set(gca,'XTick',[],'YTick',[]);title('Peak wavelength [nm]');c4=colorbar;