So first of all, the reason police ask you questions is to get the evidence they need to convict you of something. I stand by my statement of "Never talk to police!"topic7032.html
Now the charge you are facing is something like "driving with handheld device", not "driving with purple handheld device". In cross examination, you must bring reasonable doubt to the officers testimony of "saw him with cellphone in hand". If you ask officer the color and they do not know, this may weigh a tiny little bit into the reasonable doubt territory of the officers testimony but certainly, by itself, it is almost worthless. If the officer was driving in the opposite direction you can ask questions like "how far away were you when you alledge I was talking on my phone" and "how fast were you going". So for example, let's say the officer testifies that he "saw you on the phone and saw your lips moving and double checked to make sure it was a phone in your hand." So this is pretty bullet proof testimony by itself. But now let's say you confirm that the officer was driving in the opposite direction to you and you ask how far a way he was when he saw you and he says 20ft and you confirm the speed limit as 80kmh. So two vehicles moving towards each other at 80kmh means he has 0.4 seconds from 20feet before he passes you. It would be very hard for him to see you and see your lips moving and double check to make sure it was a phone in your hand in 0.4 seconds. So this an example of how you could try to bring reasonable doubt to the officers testimony.
Now with regards to what you said ... if the officer is on the witness stand and starts to say something that you said, you can object immediately and say that statement is not admissable. Two things could happen: (i) the jp will agree with you and officer will not be able to say anything that you said and the trial will continue, or (ii) the prosecutor will ask for a voir dire (trial within a trial) to find out if the statement was given voluntarily or not. This means they are going to put you on the stand to determine if you voluntarily gave the statement to the officer, or if you gave it because you thought you were required by law to answer the question. So if your testimony is what you said above that you were just trying to be co-operative, then will most likely decide that it was voluntary and not compelled and will then allow it. You want to hope for scenario (i) obviously.
The other thing to remember is that after the officer testifies and you have cross examined him, the JP will then ask if you want to call any witnesses. In this case your only witness is you, but you have the right NOT to incriminate yourself, so in this case you want to say NO and you do not want to get on the witness stand. If you get on the witness stand you must answer truthfully. All they have to do is ask you "were you talking on your phone" and you would have to say yes and you have lost. So you do not have to answer any questions put to you when you are not on the stand, and you do not have to take the witness stand at all if you do not want to.