Some times that is the best way to adjust the colors as well, I recommend choosing a similar color to stitch out on as the back ground fabric can make a difference in the all around look, they will adjust colors but you don't need to stick to them, infact the odds are that you won't have all the colors or the brand that they suggest and if they did it out in white or off white (which most of the samples that I have seen are just so) it will need a change probably if you stitch it on a dark, I have about 400 colors and there are times that I can't find the color that I would like, you can also match fabrics that you are using if you are putting it in a quilt or other project that has more than the one that you are working on,
If there are any new to embroidery users reading this, I like to also practice on tea towels and things like that, if they work out (you can do some of your practice runs on them as well) then they make good gifts, if not then you can use them yourself in the kitchen or as rags,