Brave New World satirizes the modern world's desire to use technology to take all the pain, mess and suffering from life. From the very opening of the novel, we see that every attempt is made to isolate humans from the chaos of natural life. This starts from conception. To avoid the uncertainty, pain and mess of human procreation, pregnancy, and childbirth, embryos are produced in a lab. The lab has to be kept warm for the process to work, hence the "tropical heat." However, the chilly appearance of the room represents the coldness and sterility of how life is produced--and lived--in this brave new world. Everything in the lab, as in the rest of this society, is clinical and sanitized. We learn that
The overalls of the workers were white, their hands gloved with a pale corpse-coloured rubber. The light was frozen, dead, a ghost.
Even the eggs being fertilized, which do have some sense of life, are described as uniform ("streak after luscious streak"):
Only from the yellow barrels of the microscopes did it [the light] borrow a certain rich and living substance, lying along the polished tubes like butter, streak after luscious streak in long recession down the work tables.
There is life in the embryos being formed in this sterile room, but it will soon be conditioned and emotionally numbed for the sake of its own "happiness" and an orderly society.
The fertilizing room is described in the very first passages of the book. Its description very much sets the mood for the book and tells us important things about the way the world is in this story. This is why it looks so cold--it's a symbolic coldness.
In our world, reproduction is anything but cold. It is full of passion and pain and emotion. But in this world, reproduction is carried out in
... the glass and nickel and bleakly shining porcelain of a laboratory.
Right away, we see that this is a very different society. It is one that has taken much of the humanity out of life. It has taken something as intimate and passionate as reproduction and made it into a cold, scientific procedure.
The room, then, is cold to show us how emotionally cold this "brave new world" is.