Nah, under magnification they all look that way. Try you own emitter and snap a macro picture of it . Discoloration is normal on white PCB, and the pads are pre-tinned.
The U2 is more efficient, this generally is because it is a bluer tint (less phosphorous) but not always. (U3=Brightest, U2=fairly bright, T6=bright, T5=less bright...)
@Pavithra_uk: have you possibly tested output at higher currents? Do you actualy recognize any difference?
I’ve got black PCB U2 from DD. Wierd, it might happen I’ve got just T6. Anyone else got black PCB U2 drom DD?
Ah, found this: Inexpensive Sources for XML-T6 or U2 , but no final verdict on difference versus T6.
Very hard to tell the difference between T6 and U2 by looks, higher end T6 vs lower end U2 would be practically identical. DD even has the white PCB in T6 bins, so that makes it even harder.
I've got three black T6s by the way. Just FYI, the black star has weak top layer, very easy to delaminate. Don't force the pad too much after soldering.
You don’t need lightmeter if you have decend photo camera. You just have to set different short exposure times in manual mode. If you put same current and same reflector giving same runing time (led temperature) or even better pot both leds side by side then you have to see difference on the pictures. You just have to assure the same running condition to both leds. Then you can get something like this (P7 versus XM-L, identical reflectors):
I got my leds from DD, 4pcs T6 for $14.36. I thought I will never get them.
I measured all at 700mA, and got from 285 to 300 lumen. I have a reference T5 xm-l from KD, this can 270 lumen. Another xm-l (salvaged from an ebay light) can 265 lumen, probably this is a T5 too.
So I think the xm-l T6 from DD are really a T6. Unfortunately I did not order the U2 version.