What is crema?
Crema is the initial light colored liquid that comes out during an espresso extraction. It filters up, leaving a tan colored layer on top of the darker espresso below which causes that 'Guinness effect’.
Plantation Processing
‘Naturally dry’, ‘Pulped natural/semi-washed’ or ‘Honey processed’ beans generally maintain more sugar and fat which creates more crema.
Roast Date
The 'sweet spot' for a coffee post-roast varies, coffee roasted less than 72 hours before will result in an early blonding often mistaken for crema. You want some of the C02 for the emulsification of the fat, but not so much that there's no room for the coffee solids to actually extract.
Roast Color
Darker roasts bring more of the bean's natural oil to the surface which can then transfer to packaging, grinders and other equipment resulting in less overall oil/fat in the coffee grounds. This is why you see that darker roasts can often produce less crema.
Espresso Machine Tech
Pressurized portafilters aerate the coffee during the extraction giving the illusion of crema. Superautomatic machines will often utilize technology that produce the look of crema without it actually being the emulsification of the fat/oil and the C02. This makes these machines 'user friendly' but it's also kind of a hack and often doesn't taste as rich or complex as shots pulled via traditional extraction methods.