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?

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]

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];

}

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

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

`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)

Version **Swift (5.4)**

UIColor.blue.withAlphaComponent(0.5)

The shortest possible code (**Swift 5**):

view.backgroundColor = .black.withAlphaComponent(0.75)