Simply turn the resulting automation on and off to start/stop. When it’s on it fires every second and sets the light to a color, when it stops it stops at the color it’s at then you can set your own color in automation, when it starts it always starts the same color
edit: check the comment Color Loop effect for any light with color wheel selector - #4 by C.G.B.Spender
This bp is not the best tbh, but “it just works”, though I suggest you use the one I mention in 1st post which is better optimized (I get full rainbow just switching around 3 colors with it with a long delay between) and I migrated to it recently myself: Color Loop Blueprint with Configurable Colors and Transition Time