but if you have SOME and need MORE...buy from the same manufacturer...sometimes the blue of one isn't the same blue of another manufacturer.
Good advice, but I'd even go a bit further. One should always try to get as many as you think you will need in one order. Good vendors will try to make sure that all of the buttons of a specific color on that order will be without an "oddball" from a different run.
Colors can vary a bit from each manufacturing run, even from the same supplier. They all try to do the best they can in this regard, but perfect color control isn't really possible. It makes sense when you do the research on how plastic injection molding works. Basically, a smaller percentage of pigment pellets are mixed with raw (uncolored) plastic pellets, and the ratio determines the color / intensity. It's virtually impossible to mix pellets to perfect ratios, so what comes out of the molds at any given injection, tends to be an average of what actually enters the injector, and if that can vary a bit, even in the same run.
It's also important to note that home builders tend to be more sensitive to this than the industries for which these parts are designed. For them, if it's close, it's good. Color control and the challenges it poses, especially between items of different materials, is probably one of the least understood aspects of panel building.