The flag of Brunei was originally a solid yellow background until 1906 when it added two diagonal white and black striped lines to the flags. By then, Brunei was already a British protectorate since 1888.
Historia de la Bandera
1906
With the exception of the symbol, Brunei's current national flag design has been in use since 1906, when the country became a British protectorate under an agreement with the government of the United Kingdom.