A good way to check for ripeness is skin being firm and the stalks dry enough that a knife struggles to cut through them.
As you say, if you know the variety you can tell by the colour. Yesterday I harvested some unripe Uchiki Kuri, after frost had almost killed the plants, and some fruits are bright yellow instead of deep orange, so I shall eat them soon, while the half ripe and slightly orange squashes may store if kept warm and dry. Last night I ate some ripe Hungarian Blue and it is absolutely delicious, so sweet and rich.