Different computer screens, operating systems, and even different web browsers have different color characteristics, so it’s just about impossible to get a given color to look the same on every screen.
You can get much more consistent results by calibrating your display, but this will only improve your situation — the colors that others see will still be different from what you see.
Print projects should be delivered in CMYK color, which provides much more consistent results when printed. However, the colors will still look different when they’re viewed on different screens.
To learn more about RGB and CMYK colours, check out this article on our 99designs blog!