Making a forloop wait for previous code to finish before executing without a noticable delay?

I'm writing a function that changes text in TMP, and after the text has changed I want to iterate through the text up to a specific index to change the color of each character.

It seems like the initial part - changing the text- takes too long, so the forloop starts before the text is all replaced.

I don't want to use a coroutine, and I think that async/await may be the answer, but I'm struggling to find a simple example that lets me understand how to use it.

Here is my code:

 private void EnterLetter(char typedChar)
     {
         if (quoteIndex == 0)
         {
            // Commented out for clarity as irrelevent.
         }
         if (IsCorrectLetter(typedChar))
         {
              // Commented out for clarity as irrelevent.
          }
          else
         {
           if (currentQuote[quoteIndex] == ' ')
           {
               UpdateCurrentQuote(currentQuote, quoteIndex, '·');
           }
     }
     }

     private void ChangeCharColour(int quoteIndex, bool status)
     {
         if (quoteOutput.text[quoteIndex] != Convert.ToChar(" "))
         {
             TMP_TextInfo textInfo = quoteOutput.textInfo;
             int materialIndex = textInfo.characterInfo[quoteIndex].materialReferenceIndex;
             newVertexColors = textInfo.meshInfo[materialIndex].colors32;
             int CharVertexIndex = quoteOutput.textInfo.characterInfo[quoteIndex].vertexIndex;
             if (status)
             {
                 newVertexColors[CharVertexIndex + 0] = correctColour;
                 newVertexColors[CharVertexIndex + 1] = correctColour;
                 newVertexColors[CharVertexIndex + 2] = correctColour;
                 newVertexColors[CharVertexIndex + 3] = correctColour;
             }
             else
             {
                 newVertexColors[CharVertexIndex + 0] = wrongColour;
                 newVertexColors[CharVertexIndex + 1] = wrongColour;
                 newVertexColors[CharVertexIndex + 2] = wrongColour;
                 newVertexColors[CharVertexIndex + 3] = wrongColour;
             }
             quoteOutput.UpdateVertexData(TMP_VertexDataUpdateFlags.Colors32);
         }
     }



 private void UpdateCurrentQuote(string quote, int quoteIndex, char newChar)
 {
     char[] charArr = quote.ToCharArray();
     charArr[quoteIndex] = newChar;
     currentQuote = new string(charArr);
     quoteOutput.text = currentQuote;

     ReColour(quoteIndex);
 }

 private void ReColour(int quoteIndex)
 {
     for (int i = 0; i < quoteIndex; i++)
     {
         ChangeCharColour(i, false);
     }
 }

I want ChangeColour to wait for ChangeText to be completely finished changing the text before it starts, but I don't want to specify how long it should wait with a coroutine. What is the best way for me to do this?

If possible please provide a simple example, I am new to coding.