Greyscale an image via replacing pixels

I have an image that I want to greyscale. It contains only blue and white pixels and I want to use a method whereby I identify the blue pixels and make them grey. How can I do that?

So far I have got:

fit = imread('RGB.jpg');
size(fit)
imshow(fit);

So I haven't got very far. I have created a function to test if a pixel is blue. Is there any way to use this? An example of using pixels to replace would be much appreciated.

[Ashish Uthama]

Please consider uploading the image here.

Answer 1

[Ashish Uthama]

 % air code (untested) 
 red   = fit(:,:,1); 
 green = fit(:,:,2);
 blue  = fit(:,:,3);

 isblue = blue>250;

 % Use logical indexing to set the red and green channels to be the same value as the blue channel. (When R==G==B, then the color is 'gray')
 red(isblue)   = blue(isblue);
 green(isblue) = blue(isblue);

 % reassemble
 fitout = cat(3, red, green, blue)