Is this grout paintable?

Yes this is paintable but this caulk contains colorants that may stain some porous materials such as walls, wallpaper, etc. you will want to mask or shield these areas to protect from staining and test in a non-conspicuous area to see if staining will occur. So if you are going to paint a caulking it would be more cost effective and easier to buy a clear caulk and paint over that, to avoid the potential for staining.