Bounties attract serious brainpower to the challenge.
Light-caused color change of wood is commonly regarded as a problem, especially when it comes to wooden floors and furniture, but this phenomenon could be harnessed to make desirable changes in wood color. Wood products are sometimes intentionally left in the sunshine to achieve the desired color, but there is no specialized, mechanical system designed for that purpose.
You could get a very precise shade of color this way by regulating radiation wavelength, intensity, and time of exposure.
Requires no chemicals to bleach or darken the wood.
For some types of wood (e.g. teak), a specific color tone could be achieved only by exposing it to light.
You don't necessarily need much time to get the color change of wood when exposing it to light (meaning not necessarily high energy consumption), a few days in the sun could do and when it comes to artificial UV light, color change can be achieved faster than in the sunlight.
With lamps, the light intensity can be boosted and the process doesn't depend on weather and day/night cycles.
How it works:
The mechanics behind the phenomenon: UV spectrum is responsible for most of the light-caused wood color change , but other parts of the EM spectrum can also influence the process . Photo-irradiation-caused color change of wood is mostly due to the degradation of lignin and the increase of carbonyl groups in light-treated wood . Wood can both lighten and darken when exposed to light, depending on its type.
Aging wood with lamps is not unheard of, but only as a DIY approach, I haven't found any sources mentioning this method being used in the wood industry. I also didn't find any attempts to automatize the process more than putting a piece of wood under the lamps.
What I propose: envision a (UV) lamp that automatically moves back and forth along the wood surface as if inside the scanner. This way you could keep the light source very close to the surface of the wood (requires less time and energy to get the end result) and maintain an even "tan" of the wood.
There's a big, elevated platform to put wooden boards and other wood products of various shapes and sizes on. A worker lowers the "scanner" lamp to the desired distance from the wood surface, sets the timer, and turns on the machine. The width of the light beam is also adjusted according to the width of the wood product. After some time, one tests the color of the wood and continues if needed.
The machine could also automatically turn the wood product to "tan" different sides of it. Alternatively (or additionally), a few lamps operating at once from different sides could be used. If necessary UV light can be complemented with other parts of the EM spectrum to achieve better results. Such machinery could be adapted for both industrial use and more domestic settings of small-scale woodworkers and carpenters.
Automating the system even further: after a period of rather manual working and testing (how long, what wavelengths and intensity of light it takes to achieve a certain shade for certain wood types) all the acquired data could be entered into a specialized database and the system would adapt automatically, you'd just need to enter the wood type/age and desired end color. The system could take note of the initial color of the wood product with a photo camera and change it automatically according to the desired end result color from photo examples that the person provides (one could search for those in the mentioned database).