In Regulation (EU) 2019/2015 the word monochrome is used, with illustrations in Annex III and VII of the intended labels and arrows to be used. The pictures suggest that monochrome refers to black on white background but the term 'monochrome' does not exclude that different shades of black and white could be applied, provided that the information remains clearly legible.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.