Do you mean, a cast like this (the image to the left)? A scanned slide from 1960:
Aperture has many adjustments that can help with correcting colors - if it is a systematic shift, you can get a basic correction with the white balance tool (set to natural gray) - that is what I used in the example to the right.
And then you can fiddle around with fine tuning the curves for all color channels. If all slides are corrupted in the same way, you can define an adjustment preset, that will be applied, when you import the images to Aperture.
But usually it is preferrable to do the basic color correction right in the scanner software before importing into Aperture - that might give a better photometric resolution.
Yes that is very similr to the cast I am experiencing. Your edited version is much better and I would be happy with that so thank you for your suggestion. When I said that I had scanned the slides I may have misled people as they are not scaned as such but I have set up a rig so that I can photograph them with my Canon DSLR with a macro lens. This technique works well and with practice I am now getting some pretty good results. Basically you have a light, (I found that my iPad with a torch app is excellent for providing the light source) in fronty of this you need some sort of slide holder and then the camera. The slide needs to be about an inch away from the light source and the camera as close as you can get to still be able to focus. A macro lens is best but it could be done with an ordinary lens I think. Ligning up and focussing are the biggest problems but once you have ligned the camera up, a tripod is really essential for this, you can leave it. Focussing needs care amd manual focusing is best, but with some of the darker slides it is difficult to see enough to really get pin-sharp results. But overall I am pleased with what I am manageing and it is a lot cheaper than buying a film scanner! I hav'nt tried negatives yet but will do so once I have got all my slides done.
I like your idea to use the iPad torch app as a light table
I'll have to try that myself! I am tired of the endless scanning time with my slide scanner.
One more thing I found useful for image restauration when dealing with old scans - that might help with your setup, too:
The "Curves" adjustment brick will let you change the slope of the characteristic curve. You will notice, that the adjusted image above now has a slight green tint in the lighter areas (the marble is green, not white) and not enough green in the darker areas - the leaves. You can raise the curve for dark green tints and lower it for the highlights, for example
If all fails, you can use the brushes to locally brush in color adjustments.
Yes that does make a slight difference, tweaking the image with curves is something I do ocassionally.
The set-up I have is working well but it does look a bit of a "Fred Carno's" bodge up :-) I really ought to do some work on it and make it look better but as it works fine at the moment it really is not worth the time. The slide holder was made with a great helping of triel and error and that is the weak point where I could make a proper slide carrier, Perhaps I will sometime. If you want more details let me know and I will photograph it for you.
This is best done with scanner software, to use a Command to Restore Color at the time of the scan. Most good models of HP and Epson flatbed scanners with top lighting built in have such software and commands. When this has not been done, I only have experience with Photoshop.
Typically that software has been programmed to recognize specific patterns of fade and color change. Does a beautiful job.