What format did you save them in? If you saved them as ptg files they shouldn't have changed, although you can change the canvas settings by opening the painting file in Artrage, then right clicking the layer with the canvas ( usually the base layer ), going to Edit Paper Settings, and changing the canvas to the look you prefer.
If you saved them as jpegs, it may be that the jpeg compression ( jpegs use a lossy compression method, so will tend not to look as clear as the other available formats exportable from ArtRage ) is causing the canvas to look at a little different.
One important note. It's always worthwhile saving your ArtRage paintings in PTG format ( normal painting format ) via File, Save or File Save As. The Export option is not intended to save working files, and if you need to make any changes later but have not saved, only exported it, you will not have access to a lot of the data like paint thickness, wetness, potentially layers, or even have the image at its original quality because ArtRage's native format is the only format which contains all that data.
Moving this to the tech support forum. Let me know how you get on and I hope that's of some use.