Hello RePete,
Welcome to Vista Forums!
The color change you noticed is perfectly normal when the drive approaches its full capacity (in percent of total disk memory) on your Vista machine; I've got two that are both red on my machine but still have around 10GB left on each.
In order to free up some space on your drives, consider burning your media to cd's or dvd-r's; the first can hold about 800 MB of data, and the latter 4.3 GB. DVD Dual Layer discs are expensive, but can hold up to 8.5 GB (retail value must be adjusted because they count their units "conveniently" to make it appear they have more room than they actually do). You can probably find DL's at Sams Club for around $50 for a 50 case of Verbatims; that's the best deal I've found so far.
Other options are to buy an external hdd, like a Western Digital or a Seagate. These can be purchased for around $100 and can hold anywhere from 250 to 500 GB of data.
One more option, although not my favorite, is to compress (archive) your data with a program like winrar to make it take up less room; these programs use algorithms to shrink your files, although you must unpack them whenever you want to access them later on. This process can take a considerable amount of time, especially for larger files. It is the one free option I can think of at the moment, though.
Hope that helps,
Ben