The colours on your screen are made up of RGB light, while printed jobs are made up of CMYK ink. The colours displayed on your screen is not an accurate representation of how the colours in your job will print. So please always choose your colours from a Process / Solid to Process Pantone book to give a predictable result.
The colours on your artwork might be RGB instead of CMYK. RGB colours used will be converted to CMYK in our workflow, and the resulting colour cannot be accurately predicted.
Finally, your monitor may not be calibrated correctly. Please ensure your monitor is calibrated to give the best guide possible when designing your job, otherwise you may get a completely different result once printed.
Learn more about RGB colors here.