Set background color for only part of UIView

Yes, as you are already overriding drawRect method, this will do.

- (void)drawRect:(CGRect)rect { 

    CGRect topRect = CGRectMake(0, 0, rect.size.width, rect.size.height/2.0);
    // Fill the rectangle with grey
    [[UIColor greyColor] setFill];
    UIRectFill( topRect );

    CGRect bottomRect = CGRectMake(0, rect.size.height/2.0, rect.size.width, rect.size.height/2.0);
    [[UIColor redColor] setFill];
    UIRectFill( bottomRect );

}

Change the values inside the frames as you wish.

With Swift 5.1 and iOS 13, you may choose one of the two following ways in order to solve your problem.

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#1. Draw and fill a specified CGRect instance with a UIColor instance inside a UIView subclass using UIRectFill(_:) function

UIKit provides a UIRectFill(_:) function. UIRectFill(_:) has the following declaration:

func UIRectFill(_ rect: CGRect)

Fills the specified rectangle with the current color.

The following Playground code shows how to use UIRectFill(_:):

import UIKit
import PlaygroundSupport

class CustomView: UIView {

    override init(frame: CGRect) {
        super.init(frame: frame)

        backgroundColor = UIColor.green
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func draw(_ rect: CGRect) {
        super.draw(rect)

        let bottomRect = CGRect(
            origin: CGPoint(x: rect.origin.x, y: rect.height / 2),
            size: CGSize(width: rect.size.width, height: rect.size.height / 2)
        )
        UIColor.red.set()
        UIRectFill(bottomRect)
    }

}

let view = CustomView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
PlaygroundPage.current.liveView = view

---

#2. Draw and fill a specified CGRect instance with a UIColor instance inside a UIView subclass using CGContext's fill(_:) method

CGContext has a method called fill(_:). fill(_:) has the following declaration:

func fill(_ rect: CGRect)

Paints the area contained within the provided rectangle, using the fill color in the current graphics state.

The following Playground code shows how to use fill(_:):

import UIKit
import PlaygroundSupport

class CustomView: UIView {

    override init(frame: CGRect) {
        super.init(frame: frame)

        backgroundColor = UIColor.green
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func draw(_ rect: CGRect) {
        super.draw(rect)

        let bottomRect = CGRect(
            origin: CGPoint(x: rect.origin.x, y: rect.height / 2),
            size: CGSize(width: rect.size.width, height: rect.size.height / 2)
        )
        UIColor.red.set()
        guard let context = UIGraphicsGetCurrentContext() else { return }
        context.fill(bottomRect)
    }

}

let view = CustomView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
PlaygroundPage.current.liveView = view