Why is Mars red and Venus a furnace?

Mars and Venus are Earth's nearest neighbours, almost like sister worlds. Yet one is a freezing rusty-red desert, and the other is hotter than a pizza oven, hot enough to melt lead, with a sky of crushing acid clouds. Venus is actually further from the Sun than Mercury, yet Venus is the hottest planet of all. How did three planets born from the same swirling cloud end up so wildly different?