The colour mixing results you are seeing will be a result of Artrage's gamma-corrected digital RGB colour mixing model. We are considering other more real-world colour mixing models for future versions of ArtRage.
One difference between imported images and normal media in Artrage is that imported images are brought into Artrage as thin wet paint. At some point we may look at having the ability to have variable paint thickness and other import options for external files. What you'll be noticing is that blending on layers imported from a PSD into Artrage will tend to "run out" of paint quicker due to this.