Yellow Hoard: 1941, Cat-man Comics #4 (Holyoke). A group of saboteurs wearing yellow full body hazardous materials type protective suits and hoods, they attack various plants with rifles that fire spheres of fire that also emit a deadly sleeping gas even if the fireballs miss the victim. At the Manison Steel Plant, FBI agent Craig Williams is posing as a worker when the Hoard attacks and as the Hood he unmasks one as a Chinese and gets him to talk. The man says they were smuggled in and forced to work, their leader and hangout is in a Tong Mission in Chinatown. The Hood breaks up the gang and fights their robed and hooded leader and unmasks one of them only to find that they are Chinese that have been smuggled into the country and forced in their acts of sabotage by a masked leader or be executed. Tracking the gang to Chinatown, he busts up the Tong and unmasks the leader as a Nazi posing as an Asian in order to make them carry out the mission. Interesting to note, this comic is dated December, 1941 and thus was on the stands prior to America's involvement in the War.
The Yellow Hood: 1941, Dynamic Comics #2 (Chesler). Yellow hood, gray robe, this portly villain went about killing great American leaders, coverting their heads to shrunken skulls which he delivered on to whatever appointments they had with a note: Don't wait for me any longer, here I am. The nation weakening, the great spirit Father Patriot sends his champion Major Victory to get to the bottom of it. He does, revealing the Yellow Hood as Professor Curran, a man trusted to draw up invasion plans of Japan and who had faked his own beheading in front of the hero earlier. Rather than standing trial for treason, the Hood jumped off a cliff, smashing his body on the rocks below. All things considered, could've been worse. He could have been called the Yellow Spot. Like many of the hero's stories, was reprinted in his own comic, though the Major's costume became yellow.
Yellow Spider: 1940, Champion Comics #3 (Harvey). The Yellow Spider is referenced in the blurb of issue 3 for issue 4 and is only mentioned in passing in 4 but actually appears in issue 5. I'll leave it to wiser heads as to which issue is his "first". The Yellow Spider is a dwarfish Asian who leads a whole gang of Asian men with vast resources including gas weapons, airplanes, etc. He's after Dr. Marlin's tonic that was used to make the Champ a peak of physical and mental development. To that end, the Doc's assistant was one of the Spider's men and he killed the doctor, but before he could divulge the location of the formula. He's ultimately killed by the Champ who felt bad about taking a human life though he thought it necessary and just.
Yellow Spot:1941, Dynamic Comics #1 (Chesler). Dr. Moore created an artificial man to combat the evil witchcraft of The Yellow Spot (yes, "The Yellow Spot"). The Spot, who could travel as a bat, was killing off or entrancing the minds of American scientists. He went to the home of Dr. Moore, who was creating an artificial man, and tried to stop him by stabbing him. While being stabbed, Moore somehow managed to throw the switch giving life to his Dynamic Man. Dynamic Man was able to stop the fiendish plan of the Yellow Spot and release the trapped minds of other scientists. Grabbing the bat form of the Yellow Spot, Dynamic Man proclaimed, "The Yellow Spot is rubbed from the world." The Yellow Spot wore a blue robe with a skull on the front. He was bald with just a curl of dark hair on the top of his head and was reputed as being the last of the witches of old.