function letterScore(score) {
  if (score === 'A') {
    return 'brightgreen'
  } else if (score === 'B') {
    return 'green'
  } else if (score === 'C') {
    return 'yellowgreen'
  } else if (score === 'D') {
    return 'yellow'
  } else if (score === 'E') {
    return 'orange'
  } else {
    return 'red'
  }
}
Determines the color used for a badge by matching score with grade values.
The color varies from bright green to red as the score decreases.
The score can be one of the following grade value: ['A', 'B', 'C', 'D', 'E'].
The color defaults to red if the score does not matches with any of the grade values.

@param {string} score Score value
@returns {string} Badge color