Bought used Canon Speedlite 430EX II mount flash and the plastic lens is slightly yellowed with age. Will this adversely affect the color of my digital pictures? I'm new to all this and would not know how to adjust color on my computer. Thank you so much.
It might affect the color of the flash, but flash tends to be cool in nature, and practically speaking, it may not even be noticeable.
However. A yellowed fresnel front panel on a flash head could also indicate a burned out flash tube or a flash that's been repeatedly overheated, so make sure the 430EXII you purchased is still functional. Replacing a flash tube is not something you want to attempt unless you have good experience with electronics, discharging big capacitors (dangerous voltages!), and soldering skillz.
On the other hand, this being stackexchange, if you do have said electronics, capacitor-discharging, and soldering skillz, you can easily google around and find a replacement fresnel panel for around $15-$20, and replace it if it's seriously bugging you.
Don't assume the yellowing is ageing plastic. You need to look at another unit, same make and model. Could be, the yellow nature is to filter out UV and blue light. Take some pictures and scrutinize the outcome before taking action.
P.S. Electronic flash outputs UV that cases some fabrics to reproduce with an off-color glow. Pro's often overlay the flash with Mylar plastic sheeting to minimize. Many units are fitted with UV filter and some with tinted lens to adjust color temperature to 5500K (daylight standard),
Technically, it should cause a slight warming of the pictures, emphasizing slight. However, not all bad, because sometimes a slight warming is considered a plus, makes pictures friendly, particularly portraits of people.
Indoor incandescent light is very orange, and we necessarily set Incandescent White Balance on those pictures, basically to add much blue to offset the strong orange. If we incorrectly used Daylight WB on indoor incandescent pictures, we would see a strong orange cast, very objectionable.
Human brains know to magically and automatically filter out this orange at the scene, we never even realize it is orange, but a photograph has no brain, and does capture it exactly.
We normally set Flash or Daylight white balance with flash (illuminated by only flash, or by flash and direct sunlight). But for indoor flash pictures where we want to add the existing ambient incandescent light, we would use a fairly dark orange filter on the flash (called a CTO filter, Color Temperature Orange), to make the flash match the orange incandescent light, and then we would use Incandescent white balance to compensate (to hide the orange).
If you can detect any objectionable yellow cast in your flash pictures, then it would matter. Then normal white balance correction techniques make it easy to edit a fix. See http://www.scantips.com/lights/whitebalance.html
Camera White Balance is never particularly accurate (outside in direct sunlight could be the exception), so this is something we normally always need to do, regardless of any yellowed flash lens.