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Different monitors display colors differently. In theory red should look the same on everybody's monitor, but in reality red sometimes looks pink, purple or brown.
This can be frustrating when you ask for a red, white, and blue theme and it looks purple, gray and green to you.
How can this be fixed? You need to "calibrate your monitor." This is a must before you and your web designer can be on the same page when talking about colors. And it's a good idea anyway so you can see colors the way most other people do.
There are numerous websites that help with getting the brightness and contrast set correctly (http://epaperpress.com/monitorcal/ [popup] is one example), but to get what's called your "gamma" set correctly, you'll need special software. Many monitors come with calibration software, and you can also get third-party software to calibrate your monitor. Here's a free one for Windows: http://www.hex2bit.com/products/product_mcw.asp (popup).
If you're color-blind, you might ask a non-color-blind friend to look at the colors for you.
If you really want to make it easy on your web designer, don't use the words that describe the colors like "pink", "brown", or "greenish blue". Instead use the "hex numbers."
This link shows 200+ basic colors with their hex numbers (will open in a popup): lynda.com/hexh.html (click the link called Hue then on the next page note that the hex numbers are the first ones in each box, like 990033, FF3366, CC3300, etc.).
Want to choose a color scheme yourself? Try colorblender.com. Even if you don't calibrate your monitor, if you choose the colors yourself then at least you know you're getting colors you like.