Perhaps something like this:
B = imread('peppers.png'); % Just to get a sample image
subplot(2,2,1)
imagesc(B)
[i1,j2] = ginput; % used to select a perimeter of an object...
[I1,J2] = meshgrid(1:size(B,1),1:size(B,2));
mask = inpolygon(J2,I1,i1([1:end,1]),j2([1:end,1]))'; % to create a mask
subplot(2,2,2)
imagesc(mask)
maskrgb = cat(3,mask,0*mask,0*mask); % Make an RGB-version of mask, here red
subplot(2,2,3)
imagesc((double(B)+128*maskrgb)/(256+128)) % weighted sum of images - scaled to between 0 and 1
subplot(2,2,4) % as this is what matlab expects for rgb-images in double format
imagesc((double(B)+512*maskrgb)/(256+512)) % different weighting
HTH
