I simply do not use one operating system, even device and one program to get what I need to done. I wish there was an art painting program, that is multi platform , which is made to run on OSX, Windows, Linux, iOS and Android with the same features, functions and tools.
When I start a digital painting piece, wether it turns out to be more mangerish or a digital painting, depending on the flow of things, and what I wish to express. I normally start sketching. First I search for references of poses from photographs from random pictures from searching, or if that is a hassle I pull out a traditional drawing mannequin , or load a virtual 3D mannequin , in a poser software like app on my iPad, such as Pose tool, Manga Pose Tool (http://www.alienthink.com/pose-tool-3d/) ArtPose FE
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Or even use Poser http://my.smithmicro.com/poser-3d-animation-software.html, which I do for my 3D scenes, which I now use to create entire 3D scenes as references for oil paintings, which in the past, the 3D work was my completed art, and now they are only references for my oil paintings. Before I used these apps on my tablet, I used the built-in 3D manaequins in Manga studio http://my.smithmicro.com/manga-studio-comic-illustration-software.html, or my traditional wooden one. Manga studio has a good pencil sketching tool, something which Photoshop does not have. I have tried to download brushes that would compensate, but does not have the good feel , and pressure sensitivity as the pencil stool in Manga studio, where you can do light, to hard strokes, as if sketching on paper in real life. Corel Painter http://www.painterartist.com/us/product/paint-program/, which I recently got, also has a good sketching pencil tool. Sketchbook (https://www.sketchbook.com/?locale=en), on the other hand, has a pencil tool, and it suffices with sketching as well on the computer and on the iPad. The stylus I have for the iPad is a Wacom intuos creative stylus. The problem with Sketchbook on the iPad, is it is restricted with drawing page resolutions. I always work on A3 size dimensions for better quality and detail, and if I should print it out it would look natural and less blurry and pixelated. If I were to sketch on my iPad first, it is OK, because I can always enlarge it to A3 size on my Mac. But if I sketch on my mac, and work to paint in Sketchbook on the iPad, I would get an error message stating that the artwork is too large, so Sketchbook is out as being a multi platform thing, and would use it to sketch on the iPad, to export to the Mac once only, or do a complete piece on the iPad. ArtStudio https://itunes.apple.com/us/app/artstudio-for-ipad-draw-sketch/id364017607?mt=8 is like the Photoshop for the iPad. has a terrible sketch pencil, but decent painting brushes, but can only import , and exporting is done through iTunes/email, so your stuffed if you have no internet. Then there is Medibang Paint (http://medibangpaint.com/en/) Completely multi platform, and a developing software that is like Manga Studio and free. Its sketch pencil tool sucks and is best in doing painting. Unfortunately Manga Studio is more efficient in doing line art, since the pen tool has a jitter-hand correcting settings, that makes lines neater and more precise. So Medibang would be best used for basic colouring in and painting on both platforms. If work is done on the iPad only sketches are best done first with Sketchbook , and the final work done in Medibang. The only problem is it does not seem to work well with my Stylus , with pressure sensitivity, as well as Sketchbook Medibang exports and saves are done in its own format as .mdp files, and exports to its own cloud service and storage to allow transfer between devices. If you have no internet connection, your stuffed, because it does not alliteratively save as .psd which can be imported and used. ArtStudio saves files in .art format. It also has the problem with not registering the pressure sensitivity of my stylus. Medibang paint also has an online social network site like deviant art, but in more of a google+/facebook/twitter style where you can upload final works from your tablet, or computer , or medibang cloud saves to published work, as well as it has its own ebook app to download and read the comic books and manga people created including your own. At the moment Medibang paint is not an alternative to manga studio yet, perhaps when the program evolves it may be in the future, and would be better with its extra bells and whistles of cloud storage, social networking and pulsing and selling of art and comics/manga. There is no manga studio for the tablet which is unfortunate.
When painting/clouring in and lineart I use Astropad http://astropad.com , which turns the iPad into a graphics tablet, where you can move around, like colouring on paper and visually see what you doing on the screen of the computer and the tablet.
So I mostly do my colouring in and tones and painting in Manga Studio on the Mac.
Once I have pose to work from, I mostly sketch in Manga Studio, using the Wacom Bamboo tablet (the older 2010 design)(CTH470)
Touching up work is essential after you have painted , touching up work does simple fixes such as effects to even fix proportion errors without having to scrap or redo the work. Touch ups, even can enhance colours and light tones. This is where Photoshop https://www.adobe.com/products/photoshop.html comes in, but the only problem is is its Liquify tool, which does the job but is tedious and irritating when working with layers. I have to go into windows and use corel photo paint http://www.corel.com/us/, which is an alternative to Photoshop, and its warp brush does the work, without getting confused with the layers. It also has some quick apply effects that take less steps as if you used photoshop, such as focal effects, that saves time and effort. I use my Wacom bamboo tablet for that, since Astropad only works on OSX.
Backgrounds can be painted in any program and exported in jpg format and imported into various programs , and can be moved around with iFile http://cydia.saurik.com/package/eu.heinelt.ifile/ or Photos app, if you had done them on tablet first, or you can do them on the computer. Some effects can be rendered such as space nebulas, warp fractals, slip streams and stars in native or vmware windows , and also works with wine in mac , using Apophysis http://www.apophysis.org.
So basically there is no stand alone program that does all of this on Mac and on iPad, I wish there was.