There is no direct route to officer as an armourer. However if you are a bright spark you can be selected for officer training at any stage. Bear in mind you need a degree to be a REME officer. (But you could end up as any cap badge).
As for colour blind- I'm red brown colour blind and never had a problem.
Without being harsh do you want to be an Armr or an Officer? You won't be a very good Armr if your heart isn't in it. You will delay your commissioned career if you choice to sign up as a tradesman first.
On the flip side you will generally make a better Officer if you have served as a soldier first.
You can join and train as an Armourer. You then have 2 choices...
1. If you decide you have the qualities required of an officer there is nothing to stop you applying for a commission before you're 26 (or whatever the age is; I believe the AGC will take you as a DE (Direct Entry) Officer up to 30. If you pass RCB (Regular Commissions Board) you'll be sent of to Sandhurst to do the full 9month Officer course. What you choose to be an Officer in is between you and whoever you apply to but the AFCO will give you more detail.
If you decide you want to be a REME officer they will send you to the Royal Military College of Science at Shrivenham where you will study for a degree in an engineering subject. only then will you be released on the Field Force (the proper Army) as an Officer.
2. The other choice is the stay in the ranks, if you're good enough you'll be pointed towards Artificer training. You;ll attend a Potemtial Artifcer Assessment Board (PAAB). If you pass, you'l be loaded on an Artifecer Course which last about 18 months and you leave as a Staff Sergeant. When/if you get to WO2, REME will allow you to apply for a Commission as Late Entry(LE) Officer but realistically most REME LE's are picked from the WO1s; only the most exceptional WO2s make the grade. You can off course apply for a commission in a number of other Corps as a SSgt, but again that's to wide an area to cover in this email. As well as being about 15 years away, so the rules may well have changed by then.
If you really want to do both, I'd advise joining as an Armourer; make sure you get posted to somewhere that goes to war and bang in for your commission as soon as you get your posting order for you second posting. By this time you'll have been in about 4-5 years, and should have good range of background experiences that the RCB look for. Leave it too late and you'll be too old/set in your ways.
Before you visit thew AFCO you REALLY need to decide what you WANT from your potential Army career.
Maj Boothroyd as ever is largely bang on. Some of the detail has changed slightly. Shrivenham is no longer an option, Degrees are now conducted at civilian universities under the Defence Technical Undergraduate Education Scheme (DTOES).
The AFCO is the right place to start and will be right up to date.
As a Class 3 or 2 (what you'll be initially) any deployments will be as part of your parent Unit. You'll only go off as an individual once you are Class 1. That doesn't mean though that you won't be sent to fill up another Unit that is going somewhere where the sooting is happening. Most of the individual posts don't crop up until you are Class 1. If you are serious about your career path, you'll have gone to Sandhurst long befroe your Class 1 course.
Class 3 tradesmen are not meant to work unsupervised (everything they do should be checked).
Class 2 tradesmen can work with a bit less supervision.
Class 1 tradesmen supervise 3 & 2s, and carry out inspections on equipment. I believe Class 2s can be trained as inspectors (it was a plan just as I left).
The equipment you work in is based on what you have been trained on, combined with need. If you are not trained on a piece of kit, you will generally not work on it, until you have been given at least a 5 minute brief on it...