Bokeh... wouldn't that be a secondary consideration when purchasing a lens for portraits? Wouldn't sharpness (or the absence of harshness), contrast, and color rendering be a lot more important?
Not for me--bokeh is a primary concern. All modern lenses have adequate sharpness for portraiture, and also anything that is multicoated and made since 1955 has adequate contrast. In fact, excessive contrast and excessively HIGH saturation actually work against the photographer shooting digital portraiture. A lens with a bit lower contrast is a good thing on a digital sensor, since it's super-easy to boost contrast in post. With slightly lower overall contrast, skin tones are rendered in a very flattering manner; this is the secret underlying Nikon's 105 and 135 Defocus Control lenses, which do not have the uber-high contrast of say, the new 105mm VR Nikkor or the new 60 AF-S G Micro-Nikkor.
The very-newest Nikkor lenses seem to be emphasizing high contrast and super-rich color rendition, like in the Micro-Nikkor lenses, and that tends to reveal every flaw in skin, clothes, hair,and makeup. I personally think also that many lenses which are ultra-sharp have overcorrected spherical aberration, which gives the surface "look" of ultra sharpness, a biting type of sharpness, and a harsh, awful background bokeh. A good example is the Zeiss 50mm f/1.4 Planar--horrific bokeh, extreme sharpness, and high contrast, but an ugly,sterile image.
The color rendition of the lens, if you mean the warmth or coolness of the images--that is a matter of very large consequence to many people, and the strongly yellow cast that Sigma lenses tend to render compared to Nikon glass is a good example; this is another area too where lenses tend to run in "families" and also by "generation". Sigma and Nikon within the same shoot and the same white balance do not mix well,at all.
Of course, these are my opinions. I think that lenses designed in the film era tended to focus more on different image characteristics. Sharpness and contrast often earn a lens a reputation for, well, sharpness and contrast. But those two things are, I think, less important today than they were in say 1940. "Modern" multicoated lenses have gotten better and better and have tended to be corrected to an almost sterile level, but that started to change in the 1990's with the increased emphasis on "bokeh" and highlight rendering and round diaphragms, and Nikon's Defocus Control lenses. Digital sensors are so clean and noise-free now that the slight muting effects of film grain in color negative films and higher-speed B&W films have made it so that a lens that is "too perfect" or "too good" looks very brutal on a modern higher-resolution d-slr. A good example is the Nikon 60mm Micro-Nikkor lenses of the AF-D and newer generation; ultra-high sharpness, ultra flat field, no distortion, high contrast, and high color saturation. But photographs of people made with those lenses look, well, harsh and unflattering.
The polar opposite would be the rendering of the old 58mm f/1.2 Noct-Nikkor; wider open it has veiling glare and a roundness, a softness to its images. Same with the other ultra-speed older Nikkor lenses in the f/1.2 class. ame with the 85 1.4 AF-D at its widest apertures--it is NOT sharo in the corners, and it is NOT even in illumination at the corners when wider open--it has decided fall-off. It is actually sharp in the middle, bright in the middle, soft at the edges due to a bit of field curvature, and slightly low in contrast at f/1.4 to f/2.8. And because of that, it is one of Nikon's most revered portrait lenses--its flaws are what give its images character. And the 60mm Micro-Nikkors have an awful,sterile,perfect correction look that's great when photographing one's stamp collection or dead butterflies for a scientific catalogue project, but dreadful on say a woman of 45 years of age.
Of course these are just my opinions; other people might love the uber-real or hyper-realistic rendering of perfectly corrected lenses. I prefer a more-gentle rendering that is provided by some of the newer lenses designed to offer better background bokeh, even if some rez, or contrast is a result of that decision to optimize more for bokeh and less for other criteria.