The first thing I would do is make sure you're exporting the photos with a sRGB color profile. If you use a tool such a photoshop or lightroom, your default color profile might be ProPhoto RGB which is great for print, but doesn't necessarily translate to other devices or to the web. If this is the problem, it could cause colors to appear a different hue, or even become faded when viewed on other devices.

The second thing you can try is export png, tiff, or bmp instead of jpeg. While this won't necessarily make a noticeable improvement for most photographs, it will preserve the quality of the image if your client insists on editing them herself. Each time you edit a jpeg you lose some of the detail due to the image being re-compressed in a lossy way. This does not happen with other lossless formats.

The last thing you can do is make sure the monitor you use, and the monitor your client uses is color-calibrated. Your monitor may not have come color accurate out of the box, and the intensity of the R, G, and B pixels in your display can change unevenly over time. You should re-calibrate your monitor on a regular basis, possibly monthly if you use it a lot and want your post-processing to be accurate.