Anyway, the trick is that photoreceptor cells are sensitive to a range of frequencies, with weights centered on a particular color, and return a single floating-point output. So your blue cones cannot distinguish bright green and dim blue - that's only calculated by taking the input of the green cones into account, too. See https://i.stack.imgur.com/5snTb.png for what each cone's sensitivity looks like (in this graph, lines are light received by each cone, and the colors are semi-meaningful). If the goal is to not stimulate the blue cones at all, you need to emit as close to pure red light as possible. The wavelength of your light as measured by the single average or dominant color isn't as interesting as the spectrum of all light it emits and how much that overlaps with blue sensitivity.