Originally Posted by Kitsoru
...Do you guys feel that these kinds of products are "necessary" when some color corrections are basically standard in photoshop post-processing?
Be careful on the assumption of standard, as all photogs have different styles and opinions when it comes to post processing. For a very long time I only made global changes to images in lightroom, which would not adjust for redness, blochyness, or the like. Skin requires it's own set of edits that aren't inherently part of the process, and proper makeup is always, always a better idea than hoping the photographer you don't know is decent at photoshop, whether it's for a snapshot or a full shoot.
(Your mileage per photog will vary, but that's why knowing their work helps).
As far as what they do, yes, I'm pretty familiar with with makeup products (since I've completely changed my skintone a few times now) but I don't think it's needed for a photog to know what your makeup does, especially primers, and colour corrections. Once you have it applied, it's just the canvas he's photographing. They're not really affecting the camera so much as they're affecting your skin, and proper application is the model's responsibility. @brucer007
's tips are spot on; blending is really important or you end up with a line where your makeup ends.