Water based ink durability on canvas I have been drawing on canvas using Posca, Molotow, Montana Cans and MTN markers. I like to draw using black markers and applying colors afterwards. For the background colors, I use Ecoline, both with brushes or with and airbrush to create gradients. The good thing about water-based Ecoline is that it will cover only the white/raw portion of the canvas, keeping black traces 'untouched'.

Well the issue with Ecoline seems to be that it won't last/sustain over time. In my case, after three months of keeping the canvas (picture below) in a bright varanda without direct sun light, colors are almost 100% gone. I knew this could happen, but didn't know the impact of light would be so quick and severe. It is worth mentioning that a thin layer of varnish had been applied, but apparently it wasn't effective as a means of protecting/safeguarding colors.

So my questions are: considering my style of drawing (tracing with markers first, applying colors afterwards), are there any types of inks that I could use that will be more sustainable over time than Ecoline with the same characteristics (covering white or light portions of the canvas only)? In case so, what is it and would it also work with airbrushes? Are there types of varnish or other products that could 'lock' the colors on canvas more effectively?

Or do I have to change my style and apply color first using durable ink (like acrylic), and then drawing on top of it?

Thank you very much!
Some pigments used in inks are UV sensitive and disappear rather quickly when exposed to sunlight, others remain unaffected. Having in mind the colours you need, make some research about which pigments are used to make your desired colours and which of them are UV sensitive and which are not. 

Check what water-based products are available on the market and if you can figure out which pigments they use (they are likely to be written in their catalogue which are generally available online) go for the ones you have found out that are not UV sensitive. 

Alternatively, you can buy ground non-UV sensitive pigments and learn how to make your own watercolours (follow the health and safety instructions of the tutorials, as some of the pigments are toxic). 

Since your problem is the UV light, if you want to use varnish, etc to preserve the colours you should look for something that would acts as a UV filter (without affecting your drawing).

Also, you may want to consider using oil-based markers instead of alcohol-based ones. They may do a better job at keeping your outline drawing ink/watercolour-free.