Thank you for you quick answer.
I'm expecting to receive the position x and y of the pixel of my image and his rgb hexadecimal value. I'm working with grey picture this is why I just need one color information (greyscale : r = g = b).
And indeed, I don't receive the good values. In Processing for example, in the console "color : 0000001D sent" but when I checked it on the Arduino monitor, I received a value like "434345"... Maybe it's a delay problem...
I'll try your method Robin and I'will kepp you informed.
Thank you so much !