Steve,

WRC has enough color variation for you you to use it for the base boat, light trim and dark trim.

I find it difficult to find really dark WRC, but it is there.
Unfinished boards are difficult for me to distinguish well.
One trick is to get the board sanded so its smooth and then spray with water.
It drys overnight so that won't stop any work.

I have used poplar - but I don't recommend it. Recently I looked at my original strip kayak and some isolated areas of poplar are rotted. I didn't realize it but the coating (varnish) was thin, the epoxy was locally damaged and the poplar dark and on the way to rot. Difficult to sand out - some darkness remains.
Poplar also comes in lots of shades, I got some "premium" grade from Home depot that was very white - but I've never seen any more that white.

I've used soft Maple for light trim, looked good at first by darkened to yellow fairly quickly (more yellow than the original varnish/ epoxy color)

For dark trim I use walnut. But, adjacent to an unexpected dark piece of WRC there is little difference.
Mahogany is just slightly less dark than walnut with a little red (the piece I used - all boards vary.

I agree, the easiest thing is to just do a test, but that doesn't show you the ageing problem.

Good luck