How customise Slider blue line in SwiftUI?

As pointed out in other answers you have limited ability to customize a Slider in SwiftUI. You can change the .accentColor(.red) but that only changes the minimumTrackTintColor.

Example of a Slider with .accentColor(.red)

Additionally, you can't change other things like thumbTintColor. If you want more customization than just minimumTrackTintColor that you have no choice but to use a UISlider in SwiftUI as rob mayoff stated.

Here is some code on how you can use a UISlider in SwiftUI

struct SwiftUISlider: UIViewRepresentable {

  final class Coordinator: NSObject {
    // The class property value is a binding: It’s a reference to the SwiftUISlider
    // value, which receives a reference to a @State variable value in ContentView.
    var value: Binding<Double>

    // Create the binding when you initialize the Coordinator
    init(value: Binding<Double>) {
      self.value = value
    }

    // Create a valueChanged(_:) action
    @objc func valueChanged(_ sender: UISlider) {
      self.value.wrappedValue = Double(sender.value)
    }
  }

  var thumbColor: UIColor = .white
  var minTrackColor: UIColor?
  var maxTrackColor: UIColor?

  @Binding var value: Double

  func makeUIView(context: Context) -> UISlider {
    let slider = UISlider(frame: .zero)
    slider.thumbTintColor = thumbColor
    slider.minimumTrackTintColor = minTrackColor
    slider.maximumTrackTintColor = maxTrackColor
    slider.value = Float(value)

    slider.addTarget(
      context.coordinator,
      action: #selector(Coordinator.valueChanged(_:)),
      for: .valueChanged
    )

    return slider
  }

  func updateUIView(_ uiView: UISlider, context: Context) {
    // Coordinating data between UIView and SwiftUI view
    uiView.value = Float(self.value)
  }

  func makeCoordinator() -> SwiftUISlider.Coordinator {
    Coordinator(value: $value)
  }
}

#if DEBUG
struct SwiftUISlider_Previews: PreviewProvider {
  static var previews: some View {
    SwiftUISlider(
      thumbColor: .white,
      minTrackColor: .blue,
      maxTrackColor: .green,
      value: .constant(0.5)
    )
  }
}
#endif

Then you can use this slider in your ContentView like this:

struct ContentView: View {
  @State var sliderValue: Double = 0.5

  var body: some View {
    VStack {
      Text("SliderValue: \(sliderValue)")
      // Slider(value: $sliderValue).accentColor(.red).padding(.horizontal)

      SwiftUISlider(
        thumbColor: .green,
        minTrackColor: .red,
        maxTrackColor: .blue,
        value: $sliderValue
      ).padding(.horizontal)
    }
  }
}

Example:

Link to full project

As of Apple's 2021 platforms, you can use the tint modifier to change the color of the track to the left of the slider knob. Beyond that, SwiftUI's Slider doesn't let you customize its appearance.

If you need more customization, then for now your only option is to create a UISlider and wrap it in a UIViewRepresentable. Work through the “Interfacing with UIKit” tutorial and watch WWDC 2019 Session 231: Integrating SwiftUI to learn how to use UIViewRepresentable.

The Slider documentation formerly mentioned a type named SliderStyle, but there is no documentation for SliderStyle and the type is not actually defined in the public interface of the SwiftUI framework as of Xcode 11 beta 4. It is possible that it will appear in a later release. It is also possible that we will have to wait for a future (after 13) version of SwiftUI for this ability.

If SliderStyle does appear, it might allow you to customize the appearance of a Slider in the same way that ButtonStyle lets you customize the appearance of Button—by assuming total responsibility for drawing it. So you might want to look for ButtonStyle tutorials on the net if you want to get a head start.

But SliderStyle might end up being more like TextFieldStyle. Apple provides a small number of TextFieldStyles for you to choose from, but you cannot define your own.