Judging ripeness of romanesco zucchini?

I was given some ( purchased) romanesco zucchini seeds from a friend and one grew. It has been a tasty and productive winner and I left one of my early fruits to try and ripen enough for seed harvest since it is supposedly open pollination heirloom.
My friend has grown them before but they always died early in his garden due to squash borer infection so he could not get any old enough for seed.
This one had stopped getting bigger over a month ago and the colour has changed quite strongly from the soft greens and whites of the eating fruits. I can continue to leave it but the plant is showing signs of slowing down and has mildew spots on the leaves.
Is it ripe enough to have viable seeds? Does anyone know how to tell with this variety?

We couldn't do squash in our old property so this is my first year with it and I understand the seeds are expensive so I really want to make sure it will be fully ripe.

Although I have no direct experience with Romanesco zucchini seed saving, a general rule of thumb for saving the seeds of summer squashes (including all zucchinis) is that the fruits must be left to grow until quite large with "hard-shelled rinds that cannot be dented by a fingernail". According to Seed to Seed, my main book for all veggie seed saving for over 25 years, "squashes have a greater number of viable seeds when cut from the vine and left to sit for three weeks or longer". After at least three weeks in storage, the squashes can be cut open with an axe or shovel and the flesh should be cleaned from the seeds. Then seeds are washed in a colander and all remaining string or debris is washed off. "If any squash flesh remains attached to the seeds, rub the seeds in a wire strainer under running water to loosen it. Drain and dry the seeds as with other Cucurbitaceae." I hope this helps...Good luck!

I'm growing cocozelle zucchini. It looks similar. When it's completely ripe, it's usually quite large - 40 cm long, yellow all over with faint striping and has very hard rind - harder than a butternut squash from the store. In my climate it was hardening on the vine with no water, in yours you may not want to wait too long, because it may rot. When I threw it on the compost pile and then amended the soil, a few zucchinis  germinated in random spots - so it means that seeds were good.

Well, the main plant has mostly died off and we were predicted several days of rain so I have picked my zucchini. ( cat included for scale)  It has now gone into the cold room to rest for the 3 weeks and then I'll crack it open and see what the seed situation is.

Well, it certainly looks ripe...Thanks for the update!

It appears to have been successful! The shell was hard enough that it took a cleaver, and my fathers woodchopping skills, to get it open.
I've got some flesh cooking as soup and a bit in the oven, to test if it is still edible.
Overall, fewer seeds than I expected from a squash this size, but a decent amount. I'll do germination trials in the new year.

Awesome, thanks again for the new update. Seed saving is fun once you get the basics down. Your posts are also a good reminder that I need to select a zucchini for seed saving purposes pretty soon!