const textarea = document.getElementById('yourTextareaId') as HTMLTextAreaElement;

function applyHighlight(start: number, end: number, color: string) {
    const before = textarea.value.substring(0, start);
    const highlighted = `<span style="color: ${color}">${textarea.value.substring(start, end)}</span>`;
    const after = textarea.value.substring(end);
    
    textarea.innerHTML = before + highlighted + after;
}

// forExsmple
applyHighlight(0, 10, 'red');

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