Model Railway piants, in larger quantities.

IMO, you should look for an automotive maroon that is as close as possible in your eyes. With luck you can find it in a spray can, sold for touch-up of minor scratches. Not cheap, but probably cheaper than a few dozen 30ml bottles of model railway paint.
Justification:
Just how "correct" do you want to be? NB that there's no such thing as a "correct" model railway paint. The best we can get is something that looks right, but documentary proof of the right look is always suspect, and must be interpreted. Photography does _not_ render colours as the eye sees them, nor does it render colour consistently. Not to mention the problem of colour shift in old photos.
Paint begins to weather as soon as it's applied, and over time the shade may change substantially. (Recall what happened to the blood'n'custard livery within less than a decade of exposure to the weather.) The rate and type of colour shift depends also on whether and what kind of clear overcoat (varnish) was applied, since the overcoat itself will weather. Unless you want a freshly-painted look, an exact match to the prototype's "official" colour may actually look wrong - too bright, even toylike.
Some model railway paints are allegedly matched to ancient paint chips, but that's no guarantee of correctness, since a varnish overcoat will cause the colour to look different than it does on the paint chip. And even paint chips change over time.
The lighting in our train rooms is considerably dimmer than outdoors, and also has a different spectrum, so that "correct" paints will look both too dark and colour-shifted in various ways. And the colour shift will vary between brands, since the "same colour" is often formulated differently by different companies. Generally speaking, paint applied to models for indoor use should be lighter than the real thing, in order to look right.
But if you are going to run your engine outdoors, you'll want a close match to the real colour. Do you want the just-painted look? Or the well-maintained but weathered look? They won't be exactly the same, so you have a little leeway in colour selection. That's important, for it allows you to look for a close match that looks right to you.
Hence my suggestion that you look at automotive touch-up paints.
Good hunting!

How about trying , no other connection other than a satisfied customer & I believe they sell it in big cans as well.
HTH Barry

From their site:
"Due to circumstances beyond our control (a fire in the next door unit in the early hours of the 12th July) all of our stock has been smoke damaged.
In order to maintain our high standards as the leading brand of model paints we are neither in a position, or willing to sell sub-standard products.
We have therefore decided to close for a period of time in order that new stock can be produced.
We realise that this will inconvenience some of our customers but we hope that you will understand our situation.
It is our aim to be back in production before the start of the Model Railway season in September."
I didn't realise the hobby was seasonal!

"Rich Mackin" wrote
Well it's not in my shop, but there is a distinct break in the exhibition season during the Summer, maybe that's what they mean?
John.

Thanks for the advice especially re. automotive paints. Ijust need the BS or RAL numbers for the originals. Weathering wont be a problem, the great outdoors will do that for me
I took a few pix of 20 227 at Swanwick Jct. recently & the Maroon had faded somewhat since the last Steam on the Met. weekends.
It's just possible that I might be aquiring a Metropolitain Electric next year, but that my kave to be in London Transport colours.
Thanks again, David C.

You've got a model railway in a kave? Blimey, that's a new one! How do you get mains power down there?
Cheers, Steve

Oh Dear...
Think of the last LUL "Steam on the Met" & also think garden railway... As for mains power, they would both be running on 24 volt batteries.. DC

Now you've got me reminiscing... I was at Neasden in 1965 for the big Metropolitan celebration... LT was still running a few of the old Met locos on engineers' trains. And steam engines.
Cheers, Steve