Normally Cayennes are picked after turning red but I have seen several posts saying they can be picked any time prior and usually will just be a bit less hot. I have never grown Serranos but I know they are usually green when offered in the stores. I thought that was more of a color preference thing than a taste thing. Sometimes peppers are picked for the color to determine the color of the sauce or whatever is made out of them.
I am no expert by any means but I would say you pick them when you want to. If the Serranos seem to taste "horrible" when fully ripe, then pick them before they change but if time isn't an issue, I would let the Cayenne's get red first. It seems that most peppers grow to full size and just sit there and stay green for weeks before, all of a sudden, one day you come out and look at them and you notice one or two has some orange or red on them. Then within a few more days they all start turning and soon you are pulling them off the plant a dozen at a time.
Bruce

Thanks for the advice Bruce,

I want to make dried crushed pepper from my Cayanne, and mexican green salsa from the Serrano.

But I still don't know when to harvest them, I mean, I don't even know when a pepper is "fully ripen", I am too new at this. What should be the optimal length? What's considered a full size pepper? Etc.

My mother grew seranos this year, and I harvested a number of them for her... We had some that were green, some that were red, both were fantastic. Though the red ones were a bit tougher, and substantially hotter.

A quick story - we were camping this year over new years, my sister and her family, my self, and my parents. One morning with breakfast the pepper container was out on the table, and my sister opened it up and saw a slive of a red serano on the top. She saw it and thought it was a slice of tomato (which we've never put in the pepper container but I digress)and popped it into her mouth. She exploded out of her seat and ran to the water cooler running the cold water over her burning tongue for several minutes.

We got quite a laugh out of the whole thing.

Any way the thing I think to look for in seranos and jalapenos, is when they are ripe, their skin develops what looks similar to a run in panty hose.

Here is a link that might be useful: Ripe Red Jalapenos

Everyone is right on. You can usually feel when they are ready as well( they soften up a bit). Size has little bearing. Serrano peppers will cork(stress lines or stretch marks). That is the perfect time to pick a serrano. I think when a serrano turns red it is much sweeter and milder. I prefer them green. But if you leave it on way too long they can taste bitter. As for cayenne I prefer them dark red. As soon as the pod is completely colored pick them. They began to get soften faster on the plant than serrano's.

Jamie

Once you pick your red cayennes, if you don't have a dehydrator, just string them up to dry until crispy(you may have to be patient). I've also placed them in a basket on the counter before and stirred them up once in a while until crispy dry before grinding them in my coffee grinder.

*Note:

If you plan to use the coffee grinder for grinding coffee again in the future, wash WELL in between uses.

When I pick green peppers to use, I judge by the shade of green, most usually darken up a bit when they are almost mature.
Immature peppers can taste awful.

Kevin

Here is a link:

https://www.youtube.com/watch?v=qpGqE_QMpuw