Have you ever walked into a store that sells wide screen TVs?
Ever notice how the color differs on each model. Yeah…that’s what’s happening.
Are you saving the PDFs as a raster format or a vector format out of inkscape? And why are you using .jpg format?
I’m not familiar with Inkscape but guessing the answer to your color shift may lie in the profiles you are using in both instances. Are you working in an RGB profile? A specific monitor profile? Are you saving to the same profile or something else when going to the pdf or jpg format? If you are designing something strictly for web view, you should probably work in an sRGB format all the way through. It’s limited in gamut and still doesn’t translate to CMYK printing very well but it is usually well within the gamut of most monitors. I don’t recommend sRGB format for anything but web use.
You might also want to consider a .png for web use rather than .jpg.
I can’t tell you how many times I’ve heard, “we only designed it for the web,” and how many times I’ve had to break the bad news to a client that their pretty monitor colors and low resolution raster effects are going to look completely different in print. I get that some “logos” like sigs or forum icons might only be seen online, but a lot of times, even something used on a space like Etsy wants to be printed on a T-shirt or banner…