How to get inverse color from UIColor?

This should work:

// oldColor is the UIColor to invert
const CGFloat *componentColors = CGColorGetComponents(oldColor.CGColor);

UIColor *newColor = [[UIColor alloc] initWithRed:(1.0 - componentColors[0])
                                           green:(1.0 - componentColors[1])
                                            blue:(1.0 - componentColors[2])
                                           alpha:componentColors[3]];

Source: Check if UIColor is dark or bright?

---- EDIT ----

Based on @amleszk's answer, I updated the UIColor extension/category with this method:

Swift

func inverseColor() -> UIColor {
    var alpha: CGFloat = 1.0

    var red: CGFloat = 0.0, green: CGFloat = 0.0, blue: CGFloat = 0.0
    if self.getRed(&red, green: &green, blue: &blue, alpha: &alpha) {
        return UIColor(red: 1.0 - red, green: 1.0 - green, blue: 1.0 - blue, alpha: alpha)
    }

    var hue: CGFloat = 0.0, saturation: CGFloat = 0.0, brightness: CGFloat = 0.0
    if self.getHue(&hue, saturation: &saturation, brightness: &brightness, alpha: &alpha) {
        return UIColor(hue: 1.0 - hue, saturation: 1.0 - saturation, brightness: 1.0 - brightness, alpha: alpha)
    }

    var white: CGFloat = 0.0
    if self.getWhite(&white, alpha: &alpha) {
        return UIColor(white: 1.0 - white, alpha: alpha)
    }

    return self
}

Objective-C

- (UIColor *)inverseColor {
    CGFloat alpha;

    CGFloat red, green, blue;
    if ([self getRed:&red green:&green blue:&blue alpha:&alpha]) {
        return [UIColor colorWithRed:1.0 - red green:1.0 - green blue:1.0 - blue alpha:alpha];
    }

    CGFloat hue, saturation, brightness;
    if ([self getHue:&hue saturation:&saturation brightness:&brightness alpha:&alpha]) {
        return [UIColor colorWithHue:1.0 - hue saturation:1.0 - saturation brightness:1.0 - brightness alpha:alpha];
    }

    CGFloat white;
    if ([self getWhite:&white alpha:&alpha]) {
        return [UIColor colorWithWhite:1.0 - white alpha:alpha];
    }

    return nil;
}

---- DEPRECATED ----

Based on @grc's answer, I create a UIColor category with this method:

- (UIColor *)inverseColor {

    CGColorRef oldCGColor = self.CGColor;

    int numberOfComponents = CGColorGetNumberOfComponents(oldCGColor);

    // can not invert - the only component is the alpha
    // e.g. self == [UIColor groupTableViewBackgroundColor]
    if (numberOfComponents == 1) {
        return [UIColor colorWithCGColor:oldCGColor];
    }

    const CGFloat *oldComponentColors = CGColorGetComponents(oldCGColor);
    CGFloat newComponentColors[numberOfComponents];

    int i = numberOfComponents - 1;
    newComponentColors[i] = oldComponentColors[i]; // alpha
    while (--i >= 0) {
        newComponentColors[i] = 1 - oldComponentColors[i];
    }

    CGColorRef newCGColor = CGColorCreate(CGColorGetColorSpace(oldCGColor), newComponentColors);
    UIColor *newColor = [UIColor colorWithCGColor:newCGColor];
    CGColorRelease(newCGColor);

    return newColor;
}

Swift way is to extend UIColor:

extension UIColor {
    func inverse () -> UIColor {
        var r:CGFloat = 0.0; var g:CGFloat = 0.0; var b:CGFloat = 0.0; var a:CGFloat = 0.0;
        if self.getRed(&r, green: &g, blue: &b, alpha: &a) {
            return UIColor(red: 1.0-r, green: 1.0 - g, blue: 1.0 - b, alpha: a)
        }
        return .black // Return a default colour
    }
}

iOS5+

-(UIColor*) inverseColor
{
    CGFloat r,g,b,a;
    [self getRed:&r green:&g blue:&b alpha:&a];
    return [UIColor colorWithRed:1.-r green:1.-g blue:1.-b alpha:a];
}