It sounds like the pay is kind of a seperate issue, are you sure your colleague is getting paid more than you, how do you know and have you brought this up before with anyone?
When you move to the new roles, will your colleagues pay change then? The employer won't be able to discuss this with you even if you ask them as it is confidential, and even if you asked your colleague they may say they don't want to discuss it or not tell you the truth anyway.
If you are doing similar roles both before and after with similar experience and skill sets what is the reason for the difference in pay, if there is one?
I still think your best bet for now is to take the role offered, follow the development plan and therefore build your case for increased pay whilst looking for another job, as you are planning to leave anyway soon. Also, as they are saying to do the new role equal to your colleague you must follow a development plan and that is their justification as to your different level of pay at the moment?
All other things being equal, if you are female and your colleague is male then there could be a sex discrimination angle, but if you think that might be the case then yes, I would seek professional advice as to if it would be in your interests to pursue this. Don't forget, you will need sound evidence if you are considering this option.
It is difficult to advise if informal discussion would help, without knowing the people involved. If you are fairly new and don't know them that well either, I would suggest keeping it fairly formal at the moment to avoid muddying the waters and complicating the situation any further for yourself, as they may decide you are too much hassle and want to get rid of you as I said, and you do not have many employment law rights at the moment, being fairly new to this job.
I would also be concerned about trying to push it with them when there aren't really any points to negotiate with, as I said you will need evidence and the pay of other staff is confidential information.
I'm sorry it is not really what you want to hear x