Why are optoisolators white?
Summary:


In optocoupler devices which are essentially identical apart from being either white or black in color, manufacturers'data sheets show differences in switching speed and thermal performance, white being superior in each case. 
The most notable physical parameter of the actual devices is a much lower capacitance for the white package. It seems likely that the lower capacitance is caused by a different dielectric constant in the white material and that the lower capacitance allows faster switching. 
Numerous detailed "white versus black on the same graph" comparison curves are available in the cited datasheet. 
The thermal performance of the white material is within the range that would be expected from superior radiation characteristics (and possibly also enhanced thermal conductivity.) 
Note that many early IC's were white due to the use of ceramic packages - quite different than the material being considered here. 


DATA-SHEET BASED DIFFERENCES  


Some significant to very significant differences are evident between otherwise apparently identical or nearly identical parts where the datasheet provides comparative white and black pkg data. However, some manufacturers do not offer white and black alternatives for the same industry std part numbers where others provide a choice (eg Fairchild offer white &amp; black for 4N25). 


Where a colour choice is offered, the most notable differences are  


A 1.5x to 3x improvement in switching times for white packages compared to black.
Somewhat better thermal behaviour from white packages. 

The tightness of correlation is somewhat spoiled by the manufacturers making minuscule mechanical differences which are almost certainly not relevant but which leave a very small amount of uncertainty. 


Differences which can be seen, based on real world data sheets include:


White packages have better thermal characteristics.  

Thermal resistance is lower and  

Parts can be derated at a smaller amount per degree increase in ambient temperature.  

Maximum allowed dissipation can be higher.
Input to Ouput capacitance is lower for the white package - presumably due to a difference in dielectric constant.
Switching speed is faster for the white package. Varies with load resistor. Toff affected more than Ton but both significantly difference. Ton 2x to 3x faster in White !!!


Examples of all the above can be seen in the Datasheet for Faichild 4N28 optocoupler

This version of the 4N28 can be obtained in white (with "-M" suffix) or in black. Published data sheet differences include:


Total power dissipation. 250 mW at 25C in each case but derated per degree C at   

Black - 3.3 mW
White - 2.94 mW.
DC average forward input current. Note that this seems to go against the trend but is not obviously directly thermally related. 

Black -  100 mA
White -   60 mA
LED - power dissipation and derating per degree C. Again, a "mixed message". 

Black -  150 / 2
White - 120 / 1.41.
Detector power dissipation. 150 mW at 25C in each case but derated per degree C at   

Black - 2.0 mW
White - 1.76 mW.
Input - Output isolation voltage. A bizarre result but they do seem to be differentiating. Note that 5300 VAC RMS = 7500 VAC_peak for a sine wave.  While the reason for this "different but the same" spec can be debated, it's bizarre and misleading to specify it this way. For a pure sine wave these specifications are identical but one is for 1 minutes and the other for 1 second. 

Black - 5300 VAC RMS, 60 Hz, 1 minute  

White - 7500 VAC peak, 60 Hz, 1 second
Isolation Capacitance. This seems to be significant for some applications BUT they spec each slightly differently , which prevents certain comparison.  Note that while the White value is only 40% of the Black value, which seems likely to be highly significant, the White max is 1000% o the Whitetypical but the Black typical is not stated. Very sloppy. 

Black - 0.5 pF typical

White - 0.2 pF typical, 2 pF max.
Package dimensions. Agh! Idiots. 

The black and white versions have their own package specs and there are various minor dimensional differences in many of the dimensions in each of through hole, SMD and 0.4" spaced versions. :-(.
Absolute Current Transfer Ratio - CTR.  

No differences between white and black in numeric data.

This seems wrong based on inferences which may be drawn from relative CTR data. 
Normalised Current Transfer Ratio - CTR. Idiots again, it seems.  

Graph axis are different scales (very poor practice)
Normalisation to CTR = 1 relative at 10 mA prevents full comparisons.
Black peaks higher wrt 10 mA and at lower mA than white.
Switching Speeds. Ton, Toff, Trise, Tfall. Graphs page 8. These vary with load resistance, especially Tr and Toff which are dependant on time constant of pullup resistor and device capacitance.  
At typical load resistor values (1k to 10k) Ton is about 2x to 3x faster for the white package !!!  

At 1k load Toff black is about 1.5x Toff white

At 10k load Toff black is about 2.2x Toff white

At 100k load (unusually high) Toff black is about 3x Toff white




Note: The sample size on which the above is based is impressively small. However, the differences do seem to be real and significant.