At 500px photo thumbnails are converted to sRGB, while images in individual photo view remain in the same colour space as uploaded. The colour shift can happen if the photos uploaded have not been previously exported in sRGB profile. Please note that this is not a 500px.com limitation but is a restriction placed by some browsers or devices which may not recognize the colour space in which your images were exported in.
Most browsers, applications, and devices are designed to work with sRGB, and assume that images are in the sRGB colour space. Hence, most browsers simply ignore the embedded colour space information in images and render them as sRGB. Such rendering will change colours as they appear on the original photo. It is generally recommended that if you publish your images on the web, you should always save and publish them as sRGB.
Approximate visual example:
Your colour space