Red is a colour that induces both physiological and psychological effects in humans, affecting sporting and other competitive success, signalling or enhancing male social dominance 
. There is also a broad view, based primarily on work by Masters and Johnson 
, that red, particularly red sexual skin, is indicative of sexual willingness and a defining characteristic of human female sexual arousal 
Artificial stimuli can exploit instinctive behavioural responses 
, and the link between the colour red and sexual attraction appears to be widely cross-cultural, with brothels and sex workers utilising this colour as a symbol of their trade, and lingerie, lipstick, and Valentine's Day cards predominantly featuring red as the colour of choice 
. Men appear to regard a woman as more sexually attractive and desirable if she is associated with red objects or contexts 
and it has been proposed that human males have a biological predisposition towards the colour red such that it is ‘sexually salient’ 
. It has been suggested that women seek to remind men of their red, aroused labia by displaying or wearing the colour red, especially in their use of red lipstick 
. Evidence for the importance of red in human mating behaviour and sexual interactions has come from studies of the impact of background colours 
and clothing colour 
, or discussions of female cosmetic use 
, on judgements of attractiveness, rather than images of the sexual skin for which these are assumed to be a proxy.
Humans share close phylogenetic relationships with other primate species that show enlarged, conspicuous sexual skins to signal sexual receptiveness. These swollen sexual skins occur in a variety of old-world primates; their function is not clearly understood, although a variety of hypotheses have been explored 
. Recently there have been suggestions that the size and the shape of the swellings are honest signals of female condition and genetic quality 
. For example, male chacma baboons (Papio ursinus
) prefer females whose ornaments are larger. There is also evidence that some primate males have a preference for red as the colour of these swellings. Chacma baboons show increased levels of masturbation when presented with ovariectomised females fitted with red, artificial sexual swellings 
and male rhesus macaques (Macaca mulatta
) spend significantly longer looking at red-enhanced images of the female anogenital region and surrounding skin 
. Although the function(s) of brightly coloured, conspicuous sexual skins may differ in different primate clades 
and conspicuousness of the swellings varies between individuals within a species 
, the ‘sexually salient’ hypothesis argues that, as with females in many old world primate species, women use the colour red to announce impending ovulation and sexual proceptivity, with red stimuli functioning as a proxy signal for sexually receptive genital colour. In consequence men find ‘red’ attractive and favour this colour during sexual interactions 
. There are, however, no studies in humans that parallel the tests of genital colour preference in non-human primates.
There are few systematic data concerning the characteristics of normal human female genitalia, including colour and, perhaps surprisingly given its persistence in popular science accounts, the ‘sexually salient’ hypothesis remains untested. The available evidence for this hypothesis, as summarised above, is either circumstantial or indirect, or based on simple cross-species comparisons. Only four studies have attempted to quantify variation in normal, external adult female genital morphology 
, and only two studies, since Masters and Johnson 
, have specifically studied variation in female genital colour. Analysis of Playboy Magazine centrefolds from 1957–2007 did not find any change in labia minora colour (described as pink or light red) over time, even though other features, such as amount of pubic hair, varied 
, while Lloyd et al. 
compared labial colour to surrounding skin (whether it was lighter or darker). While there is evidence for an impact of red on judgements of sexual attractiveness, it is not clear that such evidence provides support for the ‘sexually salient’ hypothesis, or how it can be reconciled with the strong cross-species evidence 
for red as a signal of social dominance.
To resolve at least part of this quandary, we present a direct test of the central assumption of the ‘sexually salient’ hypothesis, that artificial red signals are a proxy for female sexual skin, by investigating whether men's preference for red applies to the sexual skin in the vulva region. If men find women displaying red more attractive because the coloured stimuli ‘hijack’ a biological response towards female genitalia, such that men respond to it as a signal of fertility or arousal, we should see a preference for red when viewing female genitalia directly. If the ‘sexually salient’ hypothesis holds, males should either prefer redder sexual skin to other shades, or show a step increase in their ratings of attractiveness with increasing redness.