Orcs/goblins are the irredeemable villains of LOTR. In addition to attacking other races, they slay each other at the slightest provocation, and even cannibalize one another (or at least threaten to do so). The Silmarillion traces their origin back to corrupted Elves. Whereas the Elves are described as a 'totally' good race (with the exception of a few villains), the Orcs were bred by Morgoth to be the very opposite - a wholly wicked race, and LOTR does not contain a single individual who bucks the trend.
In The Fellowship of the Ring, Frodo sees the Dunlending man who has been spying on him. He is described as having "a sallow face with sly slanting eyes" and Frodo noted "He looks more than half like a goblin." In Unfinished Tales, the same Dunlending is mentioned as perhaps having Orc blood. This implies that yellowish skin-tone and slanted eyes are Orcish characteristics rather than Dunlendish traits. Creating goblins in the image of Mongoloid humans might possibly indicate some latent racial biases in British society? I've always wondered how East Asian fans of LOTR feel about this.
In LOTR, the orc-chieftain whom the fellowship met at Khazad-Dum had a "broad flat face", swarthy skin and black eyes.1 So it can be deduced that Orcs have specifically Mongoloid features such as flat faces and black slanting eyes.
In Mordor, a nameless Orc arm was described as having "dark skin of greenish scales". Ugluk the Uruk-hai was a "large black Orc".2 The Uruks were specifically identified as "black", and more evil and larger than other orcs.3 The implication is that while not all Orcs are black, they are generally darker-skinned. This is confirmed by the fact that Ugluk calls the Rohirrim "Whiteskins", which implies that the Orcs are generally not white-skinned.4 There is a general trend in LOTR of the bad people having darker coloration than the good people - red-cheeked Hobbits vs black Gollum, white Elves vs darker Orcs, white Edain vs swarthy Easterlings and Southrons.
1. LOTR, p317
2. LOTR, p437
3. LOTR, p316
4. LOTR, p441