The exercise asks us to photograph a red, yellow, green and blue subjects in a single frame, convert to black and white and digitally apply the same colour filters. With a grey card visible in part of the frame.
As you can see I chose coloured pencils and placed them onto an 18% grey material. Equipment used Nikon DSLR, 105mm Micro, ring flash and tripod.
Original in colour – as you can see red, yellow, green and blue on a grey material.
B+W unfiltered – the unfiltered image shows the colours converted to tones in proportion to the colour image.
B+W red filter – the red filter has made the red pencil tone considerably lighter, and the blue pencil much darker in tone.
B+W yellow filter – the yellow filter has made the yellow pencil a little lighter, but the others are now a similar tone to the unfiltered image.
B+W green filter – the green pencil is now lighter in tone whilst the red is darker.
B+W blue filter – the blue pencil is now close to white in tone and the red is almost black.
Apart from the yellow filter the results are as I was anticipating. The filter & matching pencil colour make that tone lighter, whilst make the others darker as more of their wavelength is blocked by the filter. Clearly from this exercise you can see how an understanding of colour for black and white photography gives you considerable control over the final image.