Cable Colour Coding

If your requirement is specific colors you cannot substitute one for the other.

What does your code say?

I went to our yard and they said we can put a sleeve on the cable with whatever color i would prefer.

Are you the original poster?

If so, sleeving may (and I say may) be allowed for the green/yellow earth but it depends on what country the equipment is going to and what that particular countries electrical regulations say.

The proper colour-coded cable is easily available and isn't expensive, why not just use that.

Probably not - even if the inspector or owner does not care to follow the requirement, or specification, or code. Maybe - if every exposed piece of the grey conductor has the Grey re-banded or shrink-wrapped or taped over with Green/Yellow and the inspector or owner does not care to follow the requirement or specification or code. No - if the intent of the design is to follow code and to build a project that follows standards and conventions to prevent mis-communication or mis-wiring, or damage to equipment or people in the future. I would be a little more inclined to substitute almost any color for any hot conductor long before I would suggest substitution of a (normally energized conductor color) for the Green/Yellow Earthing conductor. What happens in the future when additions or changes are made and the panelboards and wiring devices all have Grey conductors tied to the earthing buss bars and terminals? What if someone comes along to replace a receptacle and puts Black on hot, Grey on Hot, and Brown on Earth? What if someone is using Red/Grey/Green (after all, the project has already shown that color consistency is not a requirement) and they choose to put the green on with the blacks in the panel, since clearly grey goes on the Earth? Oh, the confusion is endless and dangerous. Stick with the color conventions for your area. I presume Europe based on your use of Cable "Colour" and color of specified cable. If this were here in Houston, there's no way you get away with that color substitution, even if the architect, engineer, designer, owner, and electricians all said " yeah, okay" because the inspectors would make the GC and Electrical subcontractor pull all that wire out and start over with the correct material.

British Standard 7671 does embrace this concept for single phase circuits. However, black as a neutral has disappeared. If one of the cores is intended to be neutral, sleeve either the black or the grey with blue and either the grey or black as green/yellow for the earth.

You are all confusing 'can' and 'should'.
I have it on good authority that electrons are colour blind... so you 'can' use any coulour you like.
However, it's a bad idea and may actually be illegal in some cases. Far better to stick to the relevant standards.

Del

If sheath of the proper material is unavailable, one can use insulating tape of the appropriate colour.

If the cable is a new addition to a pre-2004 fixed installation, a notice to the effect that the installation is wired to 2 versions of British Standard 7671 cable colours needs to be attached to the main distribution board.

All post-2006 fixed installations in UK buildings must comply with Part P of the Building Regulations in terms of inspection, testing and recording. If the new installation is reportable under Part P, stop work and consult a qualified electrician instead.

The problem here is the yellow/green conductor, which no doubt is ground/earth/PE what ever you call it in your part of the world. (forgot to see if you indicated where you are located) While most codes allow some kind of remarking such as tape or a sleeve, some don't, and most certainly some end users also prohibit it. I always check with my end user to see if they will allow this before doing so and also their local codes, because not all end users know their local codes.