How do you determine which coins are worth getting graded?..I want to get this one graded but I think the discoloration would affect the grade...
Even in slightly better condition, the coin wouldn't be especially valuable. With a mintage of 1.8 billion, it's still quite common in circulation today- sometimes in nicer shape. In order to have a valuable 1962 D cent, it would need to be at least MS64.
You would spend at least $35 to get the coin in question graded by a TPG. That is about $34.99 more than it is worth. I would go one step (or rather 2) higher than @Legomaster1 and say that you would need to have it come back at least MS66 to make it worthwhile. That is at least according to: http://www.numismedia.com/fmv/pricesms/lnccntmo/prices61.shtml You can use the PCGS photograde site to help you determine the grades on your coins: https://www.pcgs.com/PHOTOGRADE/
I personally would not send that coin in. I would, for now, use it as a place holder for your collection, do some coin roll searches to "dig" up some nicer 62-D's. Once you get some in your possession, stare and compare against your coin that you would like to get graded. That's my 2 cents (you see what I did there)
The discoloration is the least of your worries on this coin. Color isn't as big a deal (TPG's have straight graded "toned" coins since forever...) as is condition, & as already pointed out, this coin lacks the MS condition it would require to make grading feasible.
Spend some time on the PCGS website and learn. To be worth the cost of submitting, generally, memorial cents must be of VERY high grade. https://www.pcgs.com/prices/detail/lincoln-cent-modern/47/most-active