Sample code for how to highlight text within a TextField in SwiftUI in my app

text-field-with-colour.swift

TextField(placeholder, text:$event.title, axis: Axis.vertical)
    .keyboardType(.default)
    .focused($textFieldFocus, equals:controlId)

// When newline is detected, create a new item (or delete if it's empty)
// We'd do this in onKeyPress(.return) but that's only for hardware keyboards
    .lineLimit(100, reservesSpace: false)

    .overlay(alignment: Alignment(horizontal:.leading, vertical: .center)) {
        GeometryReader { proxy in
            
            if focussed.control == controlId, let timeRanges = tools.scannedTiming?.ranges {
                
                // The series of spaces is to get around the line break strategy that SwiftUI doens't give us control over
                // https://stackoverflow.com/questions/72685881/swiftui-text-doesnt-fit-full-width-while-having-multiple-lines
                // The line break strategy on TextField is different to Text. On Text it tries to avoid having orphan words
                // by having more than word fall down a line even if there's space for another above. By adding extra whitespace
                // on the end, it prevents it doing this.
                // Oh dear god of hacks this is my offering.
                Text(attributedTitle(title:event.title+"          ", highlightedRanges:timeRanges, color:.accentColor))
                    .frame(width:proxy.size.width, alignment: .leading)
                    .foregroundStyle(.clear) // to debug, .red and add .border(.red)
                    .allowsHitTesting(false)
            }
        }
    }