I think they might be OK to pick once they start to yellow and turn buff colored all over, but not quite yet, based on your photo. Seminole, if that's what it is, is supposed to be good eating so you want to make sure they are fully mature.
I was reviewing my various "lists" -- they really need to be organized better
-- and came across the list of seeds I sent you for your "garden warming"
- Squash Winter, C.mixta Cushaw White ("Jonathn Pumpkin") Baker Creek for 2013
- Squash Winter, C.moschata Seminole (x Thai Kang Kob?) Applestar Spiral Garden 2014
- Squash Winter, C.moschata Thai Kang Kob (x Tromboncino?) Applestar SFH 2015
- Squash Winter, C.moschata Tromboncino Applestar Spiral Garden 2014
…so if this possible Seminole is grown from seeds I sent you, there is a chance it was bee-crossed with Thai Kang Kob. Which may explain the somewhat not-quite appearance. It will be interesting to see if they develop any bumps or splotchy appearance in the skin. (Seminole is every colored like butternut) Thai Kang Kob will also start out dark green and smoothly ribbed but develop sort of lumpy and bumpy irregularities in the skin and become splotchy colored with blue green and yellow green -- really attractive actually -- and then turn buff/tan color. The color change will continue in storage and C.moschatas are generally considered ready to eat after one month+ in storage.
Last year when I let one change color in the garden before harvesting, the flavor was completely improved from when I had harvested it green the first year I tried growing them. Interior flesh had also deepened from yellow to orange.
For the most part though, C.moschatas are late maturing and I have to leave them in the garden until hard frost threatens. So if there are other members with experience growing them and harvesting them without worrying about frost and freeze, I would be curious to know how you determine when to harvest, too. Only test I know of is the thumbnail test
-- winter squash rind should be hard enough that thumbnail won't leave a mark.