So, um, I am a professional artist, and one of my regular tricks for figuring out why a drawing isn't working is to throw a desaturation layer on the top of the stack. Quite often it turns out that I have chosen colors that have contrast in saturation and hue, but very similar values. I play with the colors until they work in greyscale, then turn off that desaturation layer, do some small tweaks to make sure their hue/saturation work with their new values, and things look a lot better. A drawing with little contrast in its values is a drawing that is muddy and hard to make out. If the values are the same and it's high-saturation then it can even be actively painful to look at, as the colors start vibrating against each other in your visual cortex. Sometimes I do that deliberately; all rules of art are really guidelines, and any rule that says "never do this" actually means "don't do this without knowing damn well that you are going into difficult territory, and if you have a solid reason to do it anyway, make sure you're doing everything else extra well to make up for this".

Good luck making this work despite having next to no value contrast.