Wood flooring is a natural product, so no two planks are alike. Before selecting a floor we recommend viewing a few samples (ask your sales representative to show you samples specifically showing the colour and grain variation) and viewing any installation photos that are available. Remember that a Rustic floor will have more board to board colour variation than a Prime floor, but in a large area, this colour variation looks natural. A skilled installer will work from several packs, blending the floor as it's installed. Planks that don't work in the space - eg are too light or too dark may be used somewhere inconspicuous.
As well as this the wood changes colour over time, so a newly installed floor will look different from the sample, and then will look different a month after installation, and six months later etc. This isn't noticeable - the change is gradual. The floor will look more uniform the more it ages.
If the idea of a floor that's going to change over time, or the possibility of colour variation is an issue, then we would suggest a laminate flooring may be more suitable - many now are almost indistinguishable (once installed) from real wood, they are easy to maintain and as a print, they don't change colour.