Here's the problem: slides are intended to be viewed by putting light through them from behind. Your scanner is trying to bounce light off the front, though, which just doesn't work well, resulting in terrible color. What you need is a scanner with a piece of hardware called a Transparent Media Adapter or TMA. A TMA is a cover for the scanner that has a light source built in to it that shines light through whatever you put in the scanner. The reflective light on the scanner's bar is then disabled, and you can take good images of slides, backlit as they should be.
TMAs are available as an accessory for many good quality standalone scanners, but they might be pricey, especially if you have a lower-end scanner or multifunction scanner without support for an add-on TMA (in which case you would need a new scanner). So you might want to think about paying someone else to do the scanning; many camera stores offer this service.