Is it HardiePanel vertical siding or HardiePlank lap siding?

To be sure paint adheres well to ColorPlus or that an additional field applied coat of paint doesn't void the ColorPlus warranty, you should call or email James Hardie Building Products.

Here is a link that might be useful: James Hardie

yes, you can paint over Hardie. So long as the original factory applied paint is in good condition and whatever color paint you want to use will fully cover the original color(s), then there is no technical reason why you can't do this. If your contractor is planning on using a mixture of different colors of leftover boards, it might take an extra coat of paint to get the color even all over. (Just like you would have to do if you were trying to paint a single shade of blue over a room that had previously been painted in green and yellow stripes.)

The contractor's claim that this will extend the warranty by 20 years is another issue altogether! Just WHOSE paint warranty is he saying is going to be extended by 20 years? If your contractor is saying that James Hardie will then warranty the paint job for twenty years, you had better think again. The James Hardie warranty on Color Plus is only 15 years ( See the link) AND it specifically excludes "damages or defects resulting from, or in any way attributable to: ... (e) repair or alteration; ...(j) efflorescence or performance of any paints and/or coatings which are not Hardie..."

If the repainted product fails, I think Hardie could and would argue that either the repainting was an "alteration" that caused the original Color Plus to fail OR that the Color Plus didn't fail at all, your second paint job failed and that is a "coating which is not Hardie" and therefore not covered by the Hardie warranty.

If your contractor is claiming that HE will personally warranty the paint job for twenty years... well, how sure are you that he is still going to be in business (under the same name) if/when you need to collect on that warranty? If he files bankruptcy or otherwise closes down his current business, you are out of luck. Trust me, bankrupt business get discharged from "unknown" debts arising from future potential warranty claims all the time. A warranty is only as good as the business that stands behind the warranty. PERIOD.

Here is a link that might be useful: Color plus warranty from James Hardie