Can two 2500 K light bulbs replace one 5000 K bulb for growing plants indoors? In an effort to assist an old Greek woman I find myself in need of greater minds.
A 5000 Kelvin light bulb is required for her indoor fig plant. Can I get away with substituting two bulbs each in separate fixtures emitting 2500 Kelvin each? 
All answers are greatly appreciated and I'm looking forward to the education.
 A: The output of a light bulb is characterized by (at least) two parameters: the Wattage, and the color temperature.
The wattage tells you how much total energy the bulb uses (and emits). The color temperature tells you how that energy is distributed. In principle, an incandescent object emits according to Planck's Law: 
$$B(\lambda, T) = \frac{2hc^2}{\lambda^2}\frac{1}{e^{\frac{hc}{\lambda k_B}-1}}$$
As temperature gets higher, this leads to "whiter" light (more blue).
You can't just put two 2500 K lamps together to make a 5000 K bulb: you will continue to have too much red, and not enough blue.
Some attempts have been made to make light "more blue" - for example, the GE Reveal light bulb. And you can buy "grow lamps" which also are more blue.
Don't use 2500 K lamps. They will cause heating and evaporation, but lack the blue component that the plant uses for photosynthesis.
A: probably not - the 5000K is to do with the spectrum of light emitted - it will be bluer than the 2500K light. Both 2500K lights will have the a 'redder' spectrum. 
To be honest it is not even that straightforward as the temperature is an indication of the average overall temperature that the light appears to emit from. In reality the spectrum will not be broad and continuous, but rather it is likely to contain various peaks from atomic lines. The temperature is an indication of what the light will look like to our eyes. 
So I am guessing that the advice for 5000 K light is so that there is a good bit of light in the blue (and maybe a bit in the UV) for the fig plant. the 2500K lights will have some blue, but less of it. In principle you will be able to use lots of 2500K lights to have the same ammount of blue light as one 5000K light, but it will not necessarily be two lights - it might be 5 or 10. 
We can't really give a precise answer except to suggest if you can't get 5000K light you find the next closest, e.g. 4000K and maybe put in 2 or 3 instead of 1 and see how the fig plant grows.... 
A: 5000K and 2500k refer to the "temperature" of the bulb.  "Temperature" has to be in quotes because its not actually temperature, but an apparent temperature.  Yes, modern consumer lighting has gone that far off the deep end!
A bulb outputs different amounts of energy at different wavelengths.  Generally speaking, if your outputted energy is weighted towards shorter wavelengths, your light will look bluer.  If it's weighted towards longer wavelengths, your light will look redder.  From the plant's perspective, different wavelengths are processed differently by different molecules.  The plant will do its best to optimize to whatever light is available, but each species will have a preferred spectra.
Information about a spectra is hard to work with.  You could look at a plot of wavelength vs. energy emitted and try to match one chart up with another.  Fortunately, we have found that there is an easier way.  A hot object emits radiation.  The hotter it emits, the more energetic the radation it emits.  The ideal emitter is known as the "blackbody" emitter.  It emits energy with a spectra defined by the blackbody spectrum.  Very cool objects emit almost entierly in the deep infra red.  As they get hotter and hotter, they start to emit a larger fraction of their energy in smaller wavelengths.  When a hot object begins glowing red, you are actually seeing this blackbody spectra.  Hotter still the light will begin to look white, then blue.
In the past, if you wanted a 5000K light, you actually heated up a tungsten filament to 5000K, and it emitted radiation according to the spectra of a 5000K black body emitter (or rather close to it... blackbody is an ideal, not a realizable thing).  If you wanted a 2500K light, you heated up a tungsten filament to 2500K.  The result was a light which was much redder (more long wavelengths, fewer shorter wavelengths).
In this sense, you can see that adding two reddish 2500K lights is not going to produce the brighter white of a 5000K light.
Nowadays, many bulbs are fluorescent.  Fluorescent bulbs do not actually emit black body spectra, not by a long shot.  Plant bulbs try to do the best they can, but there's always funny spectral lines around the frequencies that the material fluoresced at.  While two 5000K tungsten filament lights had very similar color spectra, two 5000K fluorescent lights may have two very different ones!
For your actual question of can you replace a 5000K bulb with 2 2500K bulbs, you can see that the answer is not automatically yes because bulb temperatures do not add that way.  However, the plant may be more than happy to receive 2500K light, so you may get away with it... not because of physics but because of biology.  A plant expert would be able to better guide you as to whether the particular fig plant species in question can do well with redder light, or if the "hotter" light is essential for its metabolism.
