The best paints to use when painting pots and the tray at the bottom of box cookers is either black tempera or spray paint that states on the can that it is non-toxic when dry. In Africa blackboard paint is available and works very well.
Some house paint contains mercury or other substances as a fungicide and should definitely not be used inside an oven. Such paint gives off toxic fumes even at room temperatures and would give off even more at cooking temperatures. Nontoxic white paint has reportedly been used for inner sides in place of foil. It is important that the white be as shiny and white as possible. Such ovens have generally cooked less well than similar foiled models. Ceramic paints so far have not performed well and give off odor for very long periods of time as well as performing poorly.
For trays and the outsides of pots, any dark color will produce heat better than any light color. Light-colored or bright pots reflect solar radiation away and so are slower to heat, or heat only from the hot air with little or no heat produced on the surface of the pot. Dark red, dark green, dark blue, and dark brown colors may be used. Paints on pots are best kept thin since thick paints insulate. On surfaces darkened by handicraft methods even "nontoxic" paints still may not be safe for ingesting so food should not be laid directly on a home-painted tray but needs to be cooked inside a pot or on a commercially dark tray. Select paints that say "Non-toxic when dry" on the label (at least in the USA). Nontoxic homemade paints include children’s non-toxic black paint powder, such as Tempera. In Africa blackboard paint is used most because it chips and flakes less, which is why it is used on school blackboards, and makes it safer to cook with. Blackboard paint is available in bulk in capital cities, and tiny bottles can be obtained in most kiosks in any small village with a school.
Soot from clean wood mixed with nontoxic water-based glue or soot mixed with cooking oil that smokes at low temperatures can be used. High temperature cooking oils will remain sticky indefinitely but low temperature oils will become baked on.
Any new solar box cooker is best heated initially for several hours without food to clear out any residual vapors. Some engine and conventional black paints never get over releasing heavy odors and will ruin an oven. If in doubt, run a preliminary test before risking any questionable paint.
Any exterior paints or varnishes may be used to weather-proof the outside of the oven. The color of the outside does not affect oven temperatures if insulation is adequate. Check with your recycling center to see if there is a program in your community that lets people recycle leftover paint. You may be able to obtain free exterior paint for your cookers this way.
Pretreatment prior to painting
Beginning in 2008, the Solar Ovens team at Cornell University, Ithaca, New York performed a set of tests to compare methods of preventing paint from peeling from galvanized steel. The problem was presented by Las Mujeres Solares de Totogalpa, Madriz, Nicaragua because the paint used on the exterior and the black plate in the bottom of the solar ovens manufactured by the group sometimes flaked off or could be peeled off .
The paint used in the tests in the fall of 2008 was oil based (brand not stated). The Fall 2008 Paint Test Report recommended that a pretreatment of phosphoric acid be used prior to painting. Sanding as a pretreatment was found to be nearly as effective. The tests repeated in the spring of 2009 used a different paint, Corona brand "Dura" oil paint. Based on the Spring 2009 Paint Test Report, the results were similar with slightly less paint adhesion for all methods.
Make your own black paint
According to US patent number 401,1190 taken out by Maria Telkes of Newark, Delware in November 1975 and has since passed into public domain, a black paint that gives 13% higher temperatures compared to normal black paints can be made from 3 easily obtainable ingredients (1) Zinc Powder (2) Copper Sulphate (3) Sodium Hydroxide. Any school lab will have these and almost any town with a chemical market will have them. Or the paint can be made at a big town and distributed to the region.