Congrats on passing the exam!
Colour in music is a very odd concept - it's all about expression and imagery in your music.
There are loads of pieces of computer software around that you can enter a piece of music and variously edit it, print it and listen to it. If you enter a piece and let the computer play it back, what you will get is a numerically accurate but artistically meaningless performance. All the notes will be the right length, in the right place, at whatever speed you set. It will even work the dynamics out so that a note marked p will be played at one particular volume and a note marked f will be another, louder volume. It will be absolutely accurate, but dull.
If you then listen to a real person play it... they'll do all sorts of things to put an interpretation on it. Hang on to a note a little, bring out one note more than another, speed up or slow down a little, vary the dynamics besides what's marked... If you listen to any two pianists play the same piece it will come out differently.
You are training to be a musician, not a computer. When you play a piece, think about what images or emotions it makes you think of, and you will play it to express those. Just like when you tell someone how you feel in words, the words themselves are just sounds, it is the interpretation that both you and your listener put on them that makes them expressive. At extremes, shouting and whispering are different colours of your voice. Or, when you ask someone a question you will speak with a rising inflection at the end of the phrase. If you want to emphasise something you'll speak louder. If you want to reassure someone you'll speak gently but firmly. It's the same principle.
Colour is something you'll develop the more you play, and most of the time you won't stop and think 'oh, I coloured that piece!' - you'll just do it and be satisfied if you feel you managed to play the music with expression and meaning.
For preference you need a good teacher to help you with this - but whether or not you have a teacher, one of the most important things you can do to help is to listen to as much music as you can, not necessarily piano, but think about what you're listening to. If you listen to piano music, listen to it with the score, follow the music and think about what the pianist is playing relative to what's on the page - dynamics, tempo, pedalling, articulation... it's also useful to hear two or more recordings by different people playing the same piece and listen to the differences.