Change colour of specific frames in a avi video

Hi.

I have a .avi video with 37958 frames. I need to change the colour of some specific frames (more than 3000), for example to red or to a grayscale. Then, I have to play the movie again and watch the changed frames. I know how to process the video, but I don't know how to change the colour of specific frames. The index of the frames that I need to change are stored in the array k. Can anyone help me? This is my code:

close all;
clc
cd 'H:\Desktop\Poses_files'

%%Create a VideoReader object to read data
v = VideoReader('ines_output.avi');
vidWidth = v.Width;
vidHeight = v.Height;

%%Create a movie structure array
mov = struct('cdata',zeros(vidHeight,vidWidth,3,'uint8'),...
    'colormap',[]);

%%Read one frame at a time until the end of the video
j = 1;
while hasFrame(v)
    mov(j).cdata = readFrame(v);
    j = j+1;
end

%%Change frames with peaks
min_peaks= islocalmin(y1,'MinProminence',2);
k=find(min_peaks);

n=1;
while n<=length(k)
    mov(k(n))=   ;
    n=n+1;
end

for frame = 1 : length(k)
    % Extract the frame from the movie structure.
    thisFrame = readframe(v, frame);

end

%%Play the movie
hf = figure;
set(hf,'position',[150 150 vidWidth vidHeight]);
movie(hf,mov,1,v.FrameRate);

Many thanks,

Ines