Before you even ask, all MGA wheels were manufactured by Dunlop, and all were painted the same silver color. This includes the 1500 type disc wheels, 1600 type disk wheels, Twin Cam and "Deluxe" pin drive wheels, and all wire wheels. To the best of my knowledge, MG never installed chrome plated wire wheels at the factory. Those would all be aftermarket parts either installed by the dealer before initial sale, or by the owner at a later date. As these cars are restored, apparently chrome wire wheels are much more popular now than they ever were when the cars were younger. I personally prefer the look of painted wire wheels for nice vintage appearance, and also because I'm too cheap to pay for chrome when it doesn't make the car run any better. There was a rare factory special option for wire wheels with light weight aluminum rims, but I believe these were also painted. A modern rendition of the alloy rim wire wheel may have bare polished alloy rim and stainless steel spokes.

As with any paint job, the end result is highly dependent on initial cleaning, surface preparation, and a good primer. I wouldn't recommend using spray on lacquer from an aerosol can, as it is susceptible to attack by cleaning solvents. Ordinary enamel works okay, but takes some time to dry and cure. For better toughness you might use acrylic enamel with a catalyzing hardener, the same stuff that's common and good for painting the car body. Tougher yet might be two part epoxy urethane enamel. For the ultimate restoration (and a little more money), it is getting more popular to use baked on powder coat paint. Eastwood now has the proper powder color for the wheels. Dupli-Color also has a product called High Performance Wheel Coating which is a tough acrylic enamel formula and gives a very original color match.

With my original restoration I used "Steel-It" for the finish coat on the wheels. This is a single part air cure urethane epoxy spray paint with stainless steel pigment. It's a little darker gray like stainless steel, but tough as nails. It is also a PITA to work with because the SS pigment tends to settle out in the can during storage. If not thoroughly mixed prior to "attempted" spraying it will clog the spray nozzle badly.

But I'm sure what you really came for is the paint color code. Like paint anywhere else on the car, the type of paint is not so important as the color. Keep in mind that this is NOT a real bright silver, nothing like chrome or mirror finish. It is also not very dark gray like stainless steel shades. But in the middle it is commonly referred to as dull silver. In North America, if you walk into any automotive paint store and mention silver wheel paint, the immediate response is usually, "Ah yes, Ford Granada wheels, and the color name is Argent Silver". [Note: the North American Ford Granada (1975-1982) and the the European Ford Granada and two different models unrelated]. I will attempt to list here as many cross references to this color as possible. Feel free to suggest any more that I may miss.

Detail Silver - Eastwood
Ditzler 2593 (Argent Silver)
 "  DAR-2593 (acrylic enamel)
 "  DDL-2593 (acrylic lacquer)
Ford Silver - DAR-2593 (Ditzler Argent Silver)
Ford Silver - PPG - DCC2593 polyurethane (PPG Argent Silver)
Ford 1G - "Silver Poly" (1975-1976)
Ford 1G - "Silver Metallic" (1977-1980)
Hammered Silver - Ditzler DAR 2593
Hammered Silver - Dupli-Color® FM-149
High Performance Wheel Coating - Dupli-Color® - WP101 (silver)
High Performance Wheel Coating - Dupli-Color® - WP103 (clear overcoat)
Jewel Silver - Subaru code 9985
Metallichrome Aluminium - ICI no: P0-2358 (original paint, see note below)
Moss Silver wheel paint - 220-560 (Moss Motors Ltd)
Silver - Ditzler Delstar® - Code #16
Wheel Paint - VHT - SP188 polyurethane (Argent Silver)

Addendum, September 2012:
On 19 Sep 2012, John Hughs wrote:
"It is ICI Paints Metallichrome Aluminium part No P0-2358. I still have the original can purchased in 1964 from a friend who worked at BMC and prepared Norman Lefton's TC + others. My friend was seconded to BMC in Sodertalje Sweden to do remedial work on the imported cars including Twin Cams. The can he gave me was the exact paint used by Dunlop".

From Bob West:
"Ford 'Moondust Silver' - Ford manufacturer's code: 5SP, 6, M, O, ZJ & 2431C.
With clear over base two pack, easily available from Halfords UK in 300ml cans".

From Jim Alcorn:
"Mirror Silver then Vision Clear powder coating by Tiger Drylac".

From Realm Engineering (Alloy wheels for MGA Twin Cam and Jaguar)
"Powder Coating Silver, as per their web site".
The wheels are cast aluminum and do not look exactly like original steel wheels anyway, so this is probably not a concours issue.
On 19 Sep 2012, Adrian Cocking from Realm Engineering wrote:
"The colour ref used is called 'Silver Ash' and has a RAL ref No of 9006".

The next couple of notes apparently refer to a slightly different color.
From Peter Wood:
Reproduction 5” Dunlop Wheels – Peter said he just left it to a local paint shop to decide. Colour not recorded.
And a follow up from Jim Alcorn:
"I have used Porsche Silver and Clear for the steel spare to match Peter Wood aluminium wheels, as these are slightly brighter".