The most reliable way of painting a panel or box is to use a stove enamelling process starting with an etch primer but this requires specialist equipment and paints and is probably best left to the professionals. One-off jobs with special colours would be expensive although if you were prepared to accept a colour and finish currently being used by the supplier then the cost should be a lot less.
Cold setting acid etch primers are available from model shops and work well as long as the metal surface has been cleaned and degreased properly. I have tried “Grey Etch Primer” by HMG Paints Ltd with good results.
The next best finish uses an epoxy (two pack) paint which dries and hardens at room temperature and can be sprayed or hand painted. This type of paint is available from a number of suppliers in the UK in both primer and top coat formats with separate thinners and curing agent. The paint and hardener have a long shelf life and are only mixed in the quantities required just before use.
A do-it-yourself alternative that I have used with considerable success is to make your own near-equivalent epoxy paints and primers using a solvent based paint or primer and a small amount of pre-mixed standard Araldite or similar epoxy two-pack adhesive - I have not tried rapid-setting Araldite or oil based paints. For a primer on aluminium I used Bondaprimer with about 5-10% by volume of pre-mixed Araldite stirred into a small container of the primer paint - only use enough for the job in hand! You will have to stir the mix for some time in order to get the Araldite to fully dissolve.
Rub down the metal to be painted and thoroughly degrease and dry the surface. If there are any highlights present the paint will be rubbed off those first leaving bare metal. Support the metal off the (covered and protected) work surface using a piece or wood or similar - I use a cardboard box on its side when spraying which also helps to keep dust off the painted surface. Hand paint or spray your primer mix onto the metal and leave to dry. Clean your brush/spray-gun immediately!
To avoid the paint running, apply sparingly and leave horizontal. Make sure that the paint is dry and hard before further processing is attempted - usually 12 - 24 hours is sufficient at room temperature. It is helpful with solvent based or quick drying paints to brush the first coat on in one direction and the second coat at right angles to minimise brush marks.
When the primer or paint is dry and hard, 'Wet and Dry' rubbing down paper may be used on a flat wood block to smooth the paint surface if required (keep the surface wet with slowly running water) before the next coat is applied - thoroughly dry the work before applying further paint. If the first coat is too thin, apply another using the above procedure.
Revell colour number 48 is the closest that I have found to 'Heathkit green' for front panels. This is a matt acrylic paint that takes lettering and protective varnish well.
NB Always clean your spraygun / paint brushes quickly and thoroughly otherwise they will be unusable when the paint has dried!
Repeat the entire process for the top coat with your chosen paint - I have tried both Revell and Humbrol paints with success. You are advised to experiment with the whole process on a piece of scrap metal first. Also, shake the paint tin or mix the paint thoroughly and always use paint from the same tin on a given panel in case of minor changes in colour shades between tins.
Labelling may be carried out with Letraset or Edding dry transfer letters of a suitable size and colour - white letters on a dark paint, black letters on a light paint. You will need a guide or template to ensure that the letters are correctly positioned. I draw the required layouts on small pieces of clear film at a 1:1 scale and use that to position the letters, switch position dots, lines etc.
On completion use a fixative varnish spray (for example Letraset 101 Gloss, 102 Satin if still available or 103 Matt) to protect the lettering and stop them being disturbed. The two pack epoxy paints mentioned above are also available in a clear finish that may be used as further protection for the lettering but check that your particular type of clear varnish does not remove or affect the fixative spray or dissolve the lettering.
Clear acrylic sprays are available from car spares outlets and these seem to work well but use sparingly for the first coat otherwise it may lift or wrinkle the main colour coat or lettering. A very thin first and second coat seems to protect the underlying paint from a thicker third coat. Spraying from an increased distance produces a rougher texture finish. As always, do some tests on a scrap panel first.
With patience, the finished result looks really professional and lasts a long time.