color contrast for accessibility

You can find him at the Monash University, Melbourne, Australia.

His email address is at the bottom of this page: http://berniejenny.info/

I can't help you furthermore.

This (free) software should be mandatory on all computers : https://www.colororacle.org/

The color contrast - check manually does warrant a further check if you are trying for government level (Section 508) compliance. There is an add on that only checks color contrast. I've had to deal with this a few times lately, and even though the PDF was approved by the in-system checker, it failed the 508 compliance for contrast. 

Is the checker telling you "Color contrast - Needs manual check"? If so, this is not an error, just a friendly reminder. Color contrast, along with correct reading order and many other accessibility factors, cannot be reliably machine-checked and requires knowledgeable human inspection. Passing the Acrobat accessibility checker is a great first step, but should never be confused with 508 compliance. 

For human inspection of color contrast, a great tool is the Colour Contrast Analyser from the Paciello Group (https://developer.paciellogroup.com/resources/contrastanalyser/)

For reading order and other accessibility factors that required human inspection, a great tool is the Screen Reader Preview built into PAC3 from Access for All (https://www.access-for-all.ch/en/pdf-lab/pdf-accessibility-checker-pac/downloading-pac.html)

Both of these tools are free.