except that the led flickers and does not have a constance change of hue
So why not have a variable for the value of each colour. Then each time round the loop look at what the distance transducer is telling you (the target) and work out how far you are away from that value. Then increment the value if you are below the target or decrement it if you are above. You will do this three times one for each colour.
Then put a delay before the next loop.
The delay will control how quickly the LEDs respond.