このための utils クラスを作成することを好む人向け:
public static SpannableStringBuilder makeTextBold (Activity activity, String string, int fromCharIndex, int toCharIndex) {
    return makeTextBold(activity, new SpannableStringBuilder(string), fromCharIndex, toCharIndex);
}
public static SpannableStringBuilder makeTextBold (Activity activity, SpannableStringBuilder string, int fromCharIndex, int toCharIndex) {
    SpannableStringBuilder sb = new SpannableStringBuilder(string);
    Typeface bold = Typeface.createFromAsset(activity.getAssets(), "fonts/NexaBold.ttf");
    TypefaceSpan robotoBoldSpan = new CustomTypefaceSpan("", bold);
    sb.setSpan(robotoBoldSpan, fromCharIndex, toCharIndex, Spannable.SPAN_INCLUSIVE_INCLUSIVE);
    return sb;
}
public static SpannableStringBuilder colorText (int resourceId, String string, Activity activity, int fromCharIndex, int toCharIndex) {
    return colorText(resourceId, new SpannableStringBuilder(string), activity, fromCharIndex, toCharIndex);
}
public static SpannableStringBuilder colorText (int resourceId, SpannableStringBuilder sb, Activity activity, int fromCharIndex, int toCharIndex) {
    sb.setSpan(new ForegroundColorSpan(activity.getResources().getColor(resourceId)), fromCharIndex, toCharIndex, Spannable.SPAN_INCLUSIVE_INCLUSIVE);
    return sb;
}