I uploaded a couple of autumn landscape photos on 6 micro sites. They looked fine on SS and DT, but they looked so bad on 123, FT, and CS, that I deleted them. It seemed they had lost reds and yellows when viewed on those sites. They were edited in Lightroom 5, I usually edit them in Photoshop, could this be the problem?

Sounds like you failed to set the Color Profile to sRBG.

Agree...
I had the same problem. Most of the sites simply strip out the color managed profile and assume sRGB. If you uploaded in AdobeRGB all of your colors will appear muted online. To get the correct color the customer would have to know to open the image in a color managed program and apply (not convert) the original colorspace. Most if not all will never know this and you will most likely get fewer downloads because of the dull colors. Unfortunately the simple answer is to upload in sRGB.

I export from LR in the adobeRGB color space....I've never had a desaturated look to my images on any of the sites I have dealt with.  Check with the agencies you belong to as to their preference for color space....some prefer aRGB, some sRGB, some don't care.

The most important thing is to embed a profile in every image you create, and set Photoshop to alert you if you open images without one.

Right now, everything assumes sRGB if there's no profile or the app/browser isn't profile aware. At some point in the future that might change so you don't want sRGB to look like garbage in some future app.

I work in AdobeRGB and the action I use in Photoshop to output the JPEGs I upload converts to sRGB and switches to 8 bit before writing the file (and ensures the include profile box is checked). Likewise any save for Web presets in Photoshop or presets in Lightroom - make sure the profile is embedded and that the image is sRGB.

I used to upload aRGB JPEGs while exclusive but switched back to sRGB after becoming indie because not all sites handle aRGB (or anything other than sRGB) correctly. I think most of the sites leave a profile embedded in the JPEGs you download - 123rf used to strip all metadata, but they now leave that information alone, along with keywords etc.