but they look desaturated / discoloured / washed out
when i upload them on flickr / 500px.
This is the most common question. Don’t worry, you are just uploading your shots in Adobe RGB, the wrong Color Space for online use. Adobe RGB 1998 and sRGB IEC61966-2.1 (sRGB) are the most common working spaces used in digital photography.
sRGB (sRGB IEC61966-2.1) is a RGB color space proposed by HP and Microsoft because it approximates the color gamut of the most common computer display devices. Since sRGB serves as a "best guess" for how another person's monitor produces color, it has become the standard color space for displaying images on the internet.
Adobe RGB 1998 was designed by Adobe to encompass most of the colors achievable on CMYK printers, but by using only RGB primary colors on a device such as your computer display.
First of all set your camera to take sRGB photos. On the 5D mark III you will find the Color Space options on the 2nd submenu of the Red Camera Icon. If you're shooting RAW simply click the photo's info at the bottom center of Adobe's Camera RAW plug-in. The Workflow Options will open. Change Space to: sRGB iec61966-2.1.
Adobe Camera RAW
Adobe Camera RAW Workflow Options
If you're already in Photoshop change the photos you want to upload on flickr to sRGB by just going to Edit > Convert to Profile and choose sRGB iec61966-2.1 from the Menu. Click ok and you’re all set.
Photoshop Convert to Profile Option