Women in red-lipstick/darker shades? Can an observer of women in red lipstick or more darker shades interpret anything related to their psychology?
Rarely does one find oneself wearing lipstick unintentionally, I'd venture to guess. Therefore, one can at least conclude simply, &quot;They want their lips to look redder or darker.&quot; There is added information in this conclusion, however sparse, about the motives of such women.
You might then wonder, &quot;Why would women want their lips to appear redder or darker?&quot; You might also note that most lipsticks have this effect, or add gloss to lips that are often paler and more matte naturally. Therefore you could probably consider the function of lipstick in general, not only red and dark shades, because this already includes most. Here's a recent article you might want to check out...
Hill, S. E., Rodeheffer, C. D., Griskevicius, V., Durante, K., &amp; White, A. E. (2012). Boosting beauty in an economic decline: mating, spending, and the lipstick effect. Journal of Personality and Social Psychology, 103(2), 275–291. Available online, URLs: http://gatorlog.com/handout/handouts/Hill_Rodeheffer_Lipstic_Effect.pdf, http://personal.tcu.edu/sehill/LipstickEffectMS20March2012.pdf. Retrieved January 7, 2014.
It mostly discusses beauty products in general, and uses lipstick as a particular operationalization of bueaty products, which might even be cause for some concern about their conclusions. In any case, it should give a decent introduction to how some psychologists think about the implications of lipstick in general. I agree with @KasperBruhn that you're unlikely to hear about any good empirical research about why women choose the colors they do, but you might...This question is already the fourth hit for a &quot;lipstick shade psychology&quot; Google search! Also, the first hit is pretty informative about the psychology of men viewing women with lipstick:

Research has reasserted a strong link between males' perceptions of female attractiveness and red lipstick. Specifically, men are not only most drawn to a woman's lips, but, they spend the most time (7.3 seconds) fixated on a woman's lips when she is wearing red lipstick (as reported by the Daily Mail). Other research has similarly asserted perception of power is related to lipstick, again with red in particular.

You've got to suspect that women know that they're doing this to us...The author clearly does! As for the Daily Mail's original contribution to the discussion:

Scientists found that in the ten seconds after meeting a lady for the first time, the average chap will spend more than half his time gazing at her mouth. If she’s applied lipstick, he’ll find it difficult to look away, with a dash of pink holding his attention for 6.7 seconds and red keeping him fixated for 7.3 seconds. The study, carried out by scientists at Manchester University, involved tracking the eye movements of 50 men as they were presented with images of different women. When the women wore lipstick, the men gazed at their lips for an average of seven seconds – spending just 0.95 seconds looking at their eyes and 0.85 seconds studying their hair. But when they went without make-up, the men tired of looking at their lips after 2.2 seconds, instead devoting 2.97 seconds to admiring their eyes and 2.77 seconds to studying their noses. Full lips were deemed to be the most attractive feature, but the appeal of thin lips increased by more than 40 per cent once make-up had been applied.
Dr Geoff Beattie, who led the research, said: ‘This study proves that lips represent one of the most sensual aspects of a woman’s body and play a critical role in human sexual attraction. Full and red lips combined deliver the perfect pout to achieve male fixation, but women who simply wear lipstick – regardless of their lip type – secure significantly greater levels of attraction than those who do not. For centuries, women have painted their lips red to enhance their appeal, with this practice dating back as far as the Ancient Egyptians who commonly used red lipstick and rouge to enhance their attractiveness. The research suggests that red lips and perceived attractiveness are still inextricably linked, with red lipstick proving to be the most powerful attractor and significantly increasing visual fixation.’