I was out wandering, wool-gathering as it were, when Vicki notice the yellow Trilliums.
Rather, they appear with Red Trilliums more or less as a colour morph:
Here is a link to identifying Trilliums 101
In detail - shape and size the 2 resemble each other, too, also to opening at the same time, ahead of the White Trillium:
Vicki has seen them in other years.