Which stars are hotter: red giants or blue?

Stars are different. The giant star has a relatively low surface temperature, about 5000 degrees and a huge radius, reaching 800 solar, and due to such large dimensions, an enormous luminosity.
Red giants are stars with low temperatures (3000 – 5000 K), but with an enormous luminosity, 100 times the luminosity of the Sun. The absolute stellar magnitude of such objects is from -3m to 0m, and their maximum radiation is in the infrared range. Almost all red giants are variable stars. Inside the red giants, further thermonuclear transformation of helium takes place (helium – into carbon, carbon – into oxygen, oxygen – into silicon, and finally – silicon into iron).
The blue giant is an O or B spectral class. These are young, hot, massive stars. The masses of blue giants reach 10-20 solar masses, and the luminosity is thousands of times higher than that of the Sun.