Thanks for the report. To investigate this further, could you send an example .sketch file where you are seeing this issue over to email@example.com? In the email, please also let us know the specific elements that are showing the color discrepancies.
In the meantime, could you try going to "View > Masks" from the main menu of Axure RP, de-selecting all the options (e.g. Mask Hidden Objects, Mask Dynamic Panels, etc.), and then paste over the Sketch items using the plugin again? Does that change the behavior at all?