I have Coreopsis. I have a microwave.

I need to try this.

Two different blue flowers - nothing.
Dark maroon hollyhocks - nothing.
Sunflowers - a really attractive very pale lemon lime colored yellow green.
Red and yellow marigolds - pale yellow.

That is all the flowers I have this season. All above are bringing the temperature to near boiling and then letting the water and flower petals stand.

I got nothing with my Coreopsis. Maybe it's a certain variety?

Coreopsis tinctoria is a dye plant.

Genus - Coreopsis
Species - tinctoria

Any species called tinctoria is going to be a dye plant, i.e., Genista tinctoria yields a green color, Indigofera tinctoria yields an indigo color, Rubia tinctorum gives you a red color, etc.

Sorry for my poor explanation.