We have a similar issue logged in our tracker. Hopefully the fix will come soon.
If I understand it correctly, this happens because Corona uses Wide RGB color space internally, and this is clamped to sRGB. In the Corona VFB the colors are displayed "as expected", but if you copy the image to Max VFB or save it, it will be probably displayed a bit different, especially if there are things like colors with negative values or very high saturation.