Raunchy red or bold black?
It is impossible to determine the most sexually attractive color due to the many confounding variables involved.
Different cultures have different attitudes and preferences in the attribution of qualities of colour and behaviour. For example, "black" is associated with death in European countries. But "white" is associated with death in China and "purple" is associated with death in Brazil.
As for sexual attractiveness, there is an enormous cultural component in this as well. Gender is another confounding variable. What might be perceived as a highly sexually attractive colour on a woman may not be perceived as such on a man. And what might be a highly sexually attractive colour to a woman may not be so to a man.
Individuals associate particular colours with particular things in their lives. Thus, any colour could be "the most sexually attractive" to a particular individual. There and other confounding variables too.
Nevertheless, drawing upon various studies from the fields of colour psychology and color therapy (chromotherapy), a not-too-scientific ranking of "more" or "less" sexually attractive colors could be constructed from the list below of colors and there supposed attributes.
Black: Sophistication, elegance, seduction, mystery, sex, banishment, binding, protection, evil
Blue: Trust, reliability, belonging, coolness, tranquility, peace, calm, wisdom, justice, depression, understanding, patience, loyalty, sincerity, honour
Brown: Earthy, conservative, protective, constructive, hard-working, solid
Copper/Bronze: Love, passion, friendship, sex
Gold: Prestige, expensive, elite, authority
Green: Nature, fresh, cool, growth, abundance, rejuvenation, recovery, healing, harmony, balance, peace, hope, jealousy, envy
Grey: Neutrality, staleness, depression
Magenta: power, magnetism, spiritual power
Orange: Playfulness, warmth, vibrant, encouragement, luck, kindness, stimulation, optimism, abundance, success, independence, sensual
Pink: Soft, sweet, nurturance, security, tenderness, peace
Purple: Royal, spirituality, dignity, divination, inspiration, meditation, compassion
Red: Passion, energy, excitement, danger, speed, impulsive action, stimulation, assertiveness, aggression, strength, sex, revolt, war
Silver: Prestige, cold, scientific, stability
Turquoise: Intuitive, insightful, inventive, original, renewal, change
White: Pure, virginal, clean, youthful, mild, peace, truthfulness, enlightenment
Yellow: Warmth, cheer, happiness, vitality, change, progress, clarity, communicativeness, affirmation.
Given the above, red, black, and copper/bronze would seem to head the list of the most sexually attractive colours.
Research shows that colour affects shopping habits. Red-orange, black, and royal blue attracts impulse buyers. Pink, light green, light and navy blue attract tight budget shoppers. Pink, rose, and sky blue attract conformists.