Проблема с получением цветов фона ячейки Excel в C#

Использование RegexOptions.IgnoreCase исправит это.

public class WildcardPattern : Regex {
    public WildcardPattern(string wildCardPattern)
        : base(ConvertPatternToRegex(wildCardPattern), RegexOptions.IgnoreCase) {
    }

    public WildcardPattern(string wildcardPattern, RegexOptions regexOptions)
        : base(ConvertPatternToRegex(wildcardPattern), regexOptions) {
    }

    private static string ConvertPatternToRegex(string wildcardPattern) {
        string patternWithWildcards = Regex.Escape(wildcardPattern).Replace("\\*", ".*");
        patternWithWildcards = string.Concat("^", patternWithWildcards.Replace("\\?", "."), "$");
        return patternWithWildcards;
    }
}