SwiftUI How to change Background Color of NavigationView with Form?

You need to hide the default background of the Form to make any background colors visible.

Add this modifier to Form: .scrollContentBackground(.hidden).

So if you're aiming to make the background red, your code will look like this:

import SwiftUI

struct ContentView: View {
    @State private var input = 100.0
    @State private var inputUnit: Dimension = UnitDuration.hours
    @State private var outputUnit: Dimension = UnitDuration.minutes
    @FocusState private var inputIsFocused: Bool
    
    
    
    let unitTypes =
    [[UnitDuration.hours, UnitDuration.minutes, UnitDuration.seconds]]
    
    @State var selectedUnits = 0
    let formatter: MeasurementFormatter
    
    init() {
        formatter = MeasurementFormatter()
        formatter.unitOptions = .providedUnit
        formatter.unitStyle = .long
    }
    
    var result: String {
        let inputLength = Measurement(value: input, unit: inputUnit)
        let outputLength = inputLength.converted(to: outputUnit)
        return formatter.string(from: outputLength)
    }
    
    var body: some View {
        NavigationView {
            Form {
                Section {
                    TextField("Amount", value: $input, format: .number)
                        .keyboardType(.decimalPad)
                        .focused($inputIsFocused)
                } header: {
                    Text("Amount to convert")
                }
                
                
                
                Picker("Convert from", selection: $inputUnit) {
                    ForEach(unitTypes[selectedUnits], id: \.self) {
                        Text(formatter.string(from: $0).capitalized)
                    }
                }
                
                Picker("Convert to", selection: $outputUnit) {
                    ForEach(unitTypes[selectedUnits], id: \.self) {
                        Text(formatter.string(from: $0).capitalized)
                    }
                }
                
                Section {
                    Text(result)
                } header: {
                    Text("Result")
                }
            }
            .navigationTitle("Time Converter")
            .toolbar {
                ToolbarItemGroup(placement: .keyboard) {
                    Spacer()
                    
                    Button("Done") {
                        inputIsFocused = false
                    }
                }
            }
            .onChange(of: selectedUnits) { newSelection in
                let units = unitTypes[newSelection]
                inputUnit = units[0]
                outputUnit = units[1]
            }
            //MARK: Here's the solution
            .scrollContentBackground(.hidden)
            .background(Color.red)
        }
        
        
        
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}