I'm not sure what the output (lumens) is on the OEM HID. 4300K is likely it as it is the closest to daylight.
I got the following from OSRAM Sylvania website;
http://www.sylvania.com/en-us/innovation/education/learnautomotivelighting/Pages/FAQs.aspx
Q) How much whiter are SilverStar® ULTRA headlights?
A) SilverStar® ULTRA is up to 4100K (degrees Kelvin) on the color temperature scale, which is a whiter color that is closer to the color of daylight. Standard halogen headlights are about 3100K and have a yellowish color in comparison.
Peter
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Peter, please don't confuse output (measured in lumens) with color temperature (measured in Kelvin). I've seen you quote color temps on numerous occasions in response to a question about light output and brightness.
Brightness is measured in lumens (simplistically). The higher the number the brighter something is to the human eye.
Color temperature is measured in Kelvin. Somewhat more complicated to explain but lower numbers indicate 'warmer' hues (red/orange/yellow) and higher numbers indicate 'cooler' hues (white/blue/purple). 'Daylight' is not one specific color temperature as it varies throughout the day and depends on the weather but is generally accepted to be ~5500K.
Varying color temperature
at a specific output will affect the perceived brightness of something to the human eye, but it is misleading to state that higher (or lower) color temperatures are 'less bright' as you absolutely can have a brighter lights at those color temps. If you want to discuss brightness you need to find a measure of a bulb's output in lumens.
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At the end of the day the headlight design in this model of Explorer just isn't very good and that will be the limiting factor with regard to light output down the road and ultimate visibility.
The best option for halogen equipped vehicles is an aftermarket HID upgrade kit. I got mine from coolbulbs.com (35w 6000K) and their kit is high quality and fully plug and play. It took me less than an hour to install (and I was going slow) and I have had no problems with it whatsoever. You don't need to make any adjustments to the vehicle's wiring, simply trim the dust cap on the back on the headlight unit and find somewhere to mount the ballast and starter. Light output is similar to, if not better than, my old HID equipped 2012 Limited.
The next best alternative is an aftermarket incandescent bulb upgrade. There are lots of bulb options out there but all incandescent bulbs work on the same principal of heating a wire filament in a gas filled bulb. The light comes from the glowing wire and the gas is there to allow it to glow hotter and brighter without burning out. Making a bulb brighter requires over-driving it which reduces its lifespan because more heat is produced. Read some of the 'lighting' forums like candlepowerforums.com for some real educated input and analysis. Bulb manufacturers try and fake the look of HID bulbs by adjusting their color with bulb coatings simply because it's fashionable and perceived as 'high end'. These coatings only serve to reduce the brightness of the bulb. 'Xenon' bulbs may use xenon gas but it is a marketing gimmick used to fool buyers into thinking they are equivalent to HID bulbs in color and/or output but this is false. The light still comes from a glowing filament, not from a ball of plasma (as in true HID bulbs). As others have stated, aftermarket bulbs are a perfectly viable option and may improve light output but they can be expensive ($60+) and wont last as long as either standard bulbs or a good quality HID upgrade.
Considering my HID upgrade was the same cost as two sets of aftermarket bulbs, or approximately 2 years worth of replacements, it was a simple decision for an improved solution.
