B = Blue
G = Green
R = Red
Y = Yellow
When you see the Letter O, it means you have a correct colour in the correct position. The letter N means you have a correct colour, but it is in the wrong position. So, ONN would mean you have one colour in the right position and 2 other colours are correct, but they are in the wrong position.
Based on the old game - Mastermind. Try to arrange the 4 different "Colours" in the correct order. Colours may be repeated. Each pattern is entered as 4 letters e.g. YRBR.