It depends on how accurate and consistant your results need to be.

For consistency you will need a measured light source (expensive but less than a spectrometer). Macbeth cards may get you close enough visually

https://calibrite.com/jp/product/colorchecker-classic/

but if you are measuring colour then you need a booth to avoid different results as the ambient light changes. https://www.gtilite.com/

Brian Buxton - thanks - I'm really wondering how to extract numerical data in rgb color space from the camera histograms.

You can save a LUT (look up table) from most image editors. I wonder how consistent your colours are from shot to shot. If you photograph the same plant at different times without controlled lighting there is no baseline to measure from.

Thanks. I'll be using a copy stand with samples brought in from the field. I am hoping that Affinity has the ability to produce an LUT from a free-form crop. of the photo.

Affinity photo can export as LUT. It looks like an interesting project and for measuring the health of plants available light is probably ok. Most plants also reflect a lot in IR and UV ends of the spectrum. It is not so difficult to get a cheap digital camera and have the UV and IR filters pulled off the sensor. https://www.lifepixel.com/ this maybe useful for measuring ripeness and health of samples.