You have to decide if your image is greyscale or colour. You obviously can't set a pixel to green in a greyscale image without first converting it to truecolour. In any case, your if/else would produce a binary image which by default is displayed in black and white.
Anyway, in matlab you wouldn't use a loop for this
%greyimage: your 2d matrix of the image, most likely of type uint8 if you're using intensities 183 and 215
bwimage = ismember(greyimage, [183, 215]);
%bwimage is a logical array the same size as the image with true (1) when the pixel has value 183 or 215 and false (0) otherwise
