Adjust alpha of UIColor

I set a UIColor using rgb to a background of a UILabel. I'm trying to adjust the alpha only. How can I modify the alpha of an existing rgb UIColor?

Edit

Basically I have UILabels that have a set UIColor (using rgb), and I won't know what color the UILabels are. At a certain point, I will have to change the labels alpha`. How can I just change the labels color alpha?

colorWithAlphaComponent: did the trick.

So what I had to do was:

self.mylabel.backgroundColor = [self.myLabel.backgroundColor colorWithAlphaComponent:0.3];

Swift 3+

yourUIView.backgroundColor = UIColor.white.withAlphaComponent(0.75)

Swift 2

yourUIView.backgroundColor = UIColor.whiteColor().colorWithAlphaComponent(0.75)

If you have custom colors defined, you can do this as well:

view.backgroundColor = [[MyClass someColor] colorWithAlphaComponent:0.25];

Version Swift (5.4)

UIColor.blue.withAlphaComponent(0.5)

why not using label.alpha = 0.5 ? to adjust your label's alpha?

update: if you want to adjust alpha from a old color, here is an example:

UIColor uicolor = [UIColor greenColor];
CGColorRef color = [uicolor CGColor];

int numComponents = CGColorGetNumberOfComponents(color);

UIColor newColor;
if (numComponents == 4)
{
    const CGFloat *components = CGColorGetComponents(color);
    CGFloat red = components[0];
    CGFloat green = components[1];
    CGFloat blue = components[2];
    newColor = [UIColor colorWithRed: red green: green blue: blue alpha: YOURNEWALPHA];
    
}

The shortest possible code (Swift 5):

view.backgroundColor = .black.withAlphaComponent(0.75)

Please set alpha in below code -

[UIColor colorWithRed:200.0/255.0 green:191.0/255.0 blue:231.0 /255.0 alpha:1.0]