You can use coloured pencils or sketch pens in CBSE board exams since it isn't strictly prohibited but it is best to avoid them for while drawing diagrams or illustrations. Sticking to a regular pencil will save more time as compared to coloured diagrams. The focus should be on answering questions rather than making things look artistic.

Hello Dear,

There's no such restriction on using color pencils for boards but It is advised not to use coloured pencils for drawings diagrams. Diagrams should be drawn and labelled only with the help of a grey pencil and not even pen. In board exams students generally don't have that much time to use colors in making their paper look more attractive. They will cost you more time and there is no extra marks are given for colourful diagram.

Good Luck