My guess is that your input images have an embedded color profile other than sRGB, right? Resizer dosen't add profiles to thumbnails, in an effort to keep them as small as possible. (Some CMYK profiles are upwards of a megabyte!)
It does make an attempt to properly convert the image to sRGB when generating a thumbnail, but it can only do it if ImageMagick on the server supports profile conversion, and some installs don't (*cough* MODX Cloud *cough*). Otherwise the profile gets stripped. In order for it to work you need ImageMagick >= 6.5.9 and it needs to have been compiled with the
lcms delegate. You can check with this command in a shell:
convert -list configure | grep DELEGATES
You should see lcms listed.
Using profiles other than sRGB typically isn't a good idea on the web. Up until fairly recently only a few browsers supported them. Now most desktop ones do; don't know about mobile. Older browsers simply ignore them and visitors see the same muted colors you noticed. Search for 'browser color profile support' and look at some of the links. Most monitors can't display gamuts much larger than sRGB anyway, so Adobe 1998, Prophoto, etc. aren't doing you any favors on the web unless you cater to graphics professionals with wide-gamut displays.
When you're saving images for the web, generally you want to convert them to sRGB then not embed the profile (sRGB is assumed). That ought to avoid this problem altogether. This article is old, but has some good background information:
http://www.gballard.net/psd/saveforwebshift.html.
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