Most blonde hair extensions come in 'raw' shades which can be a little brassy. However, this is easily fixed. Simply wash the extensions in DILUTED purple shampoo, et voila!
Do this infrequently and your extensions will stay bright, and brass-free. (Don't do it too regularly or use a very dark purple shampoo otherwise you will end up with lilac-my-little-pony extensions!....unless thats what you wanted of course)