RGB vs. CMYK? Understanding the 2 is critical.
Why would the color on your monitor and the color on the actual postcard vary? Simply because they use different methods for creating color.
There are differences between what you see on your computer screen and the output of your color printer. In some cases, they can be quite different. Color displayed on your computer monitor is created by projected or additive light. Red, Green, and Blue lights (hence the term RGB) are projected in differing strengths to form various color combinations. White results from combining Red, Green, and Blue at full strength.
Printed color, however, is reflected or subtractive color. The printed inks absorb (subtract) certain colors from the visible light spectrum -- what you see is actually the light which is reflected off the printed matter (paper and inks). Printed color is referred to as CMYK color because Cyan, Magenta, Yellow, and Black inks are used to create the colors. As such, it is possible to create colors in RGB that you cannot make in CMYK. These colors are said to be "outside the CMYK color gamut". So if you are working in a photo editing program such as Adobe Photoshop, and are working in RGB mode, you can create colors that look great on your monitor, but are nonetheless impossible to print. This is because before a file can be printed on an offset press it must be converted to CMYK.
If your image editing program has a "CMYK Preview" option, you should always work in that mode. As a result of these different methods for creating color, color mismatches often occur between monitors and printers. The range of color which can be produced on a monitor differs from that of a printer -- both devices produce some colors which the other cannot. Further discrepancies occur because computer monitors are rarely calibrated to any standard settings and their colors shift as they age.