I = imread('yellowdot.jpg');
imshow(I)
A = imread('yellowdot.jpg');
Red = A(:,:,1);
Till here you are doing it correctly.
Here "Red" is a red plane of the A rgb Image, its a 2D array with datatype uint8 (Unsigned interger 8 Bits). When the data type with uint8, its have maximum value 2^8=256, if Pixels considertion satrt from 0, hence maximum pixel value is 255 in any plane for uint8 data type.
If you check the details of A, it may be shown as
A>> "288 X 384X3 uint8" for 3 channels (3D Array), once the extraction of first planes from the 3rd dimentions, it represents the red plane. Hence it may shows
Red>>"288 X 384 uint8"
2nd Section:
"When I retrieve the value no matter the x,y values i put into that last line it outputs "120"
Red(250,190) represent the pixel value/Matrix value at the incices rows 250 and column 190. May be the given image have same color irrespective of any indices location, please do confirm?
More: If you wish to check the pixel value in differrent positions of the image, you can use the following code (by moving the Mouse cursor)
A = imread('yellowdot.jpg');
Red = A(:,:,1);
imshow(Red)
impixelinfo;
:)
