Paint gradients?

[Slothboy3000]

Are there any guides that list blending/ gradient techniques with paint? I'm a fan of illustrations from the 40s and 50s where smooth paint gradients seemed prevelant and was wondering what the best medium would be to create gradients like that. Thanks :)
Here's an example of Loomis' stuff http://1.bp.blogspot.com/_A5qhnmd_hog/RtX1znaz1HI/AAAAAAAABII/Mtsu7qdLXr0/s400/loomis6.jpg