Two likely reasons. You have not assigned the sRGB colour space to the image, - your left it with Adobe RGB; and if you use Internet Explorer to view the uploaded images, - its will read the colours incorrectly.
heres a link to check your browser:
The actual conversion of the colour space should be done in Photoshop as part of the upload for web process; and the PPI should be set to 72, before you re size. Then after saving for web, - open the new image, check for sharpness, and apply as needed. re sizing can cause loss of detail due to file compression.
The image exposure looks good, - though to be honest I would have used f/8 and a faster speed, than f/18. You risked loss of sharpness, - and having to sit extremely still for 0.6 seconds!
OK, - so I modified you original shot, V3. You arbitrarily set the white balance, - and we often do this, BUT you are way better off calibrating it especially when shooting RAW. Using the white piece of paper, - set in on the chair in the same light, take a shot of it in RAW and AWB, - you may need to manual focus, - and then proceed with your session. When you process the RAW files, open the white shot first, select the white balance tool, click it on the *****, and take a look at the white balance settings. Apply these to all shots taken in the same light.
Ive obviously cropped as you did; applied quite a bit of sharpening; and the main change is in the colour tone. I have no idea what the real colour of the wall behind you is, but the technique I use suggests its more yellow than white?
heres a link to the technique:
Hope this helps, - and if it doesnt answer your question, and you still have a problem, - let us know or send me a PM.