The stencil is indeed from that one stencil site linked in the stencil thread on /m/
You can just google the name of the mecha though and stencil to find stuff tho
Why does a website not exist that can be used as a colour guide with all the colour palettes of the most common modelling paints exist
Pretty sure nobody cares enough to do it, it's a pretty big project. Plus because no paint brand carries every single tone you'd ever want, you always have to mix anyway and at that point the site's usefulness becomes so-so though I guess you could have palette options for more common mixes along with the recipes.
Plus if you want to get technical, you can't accurately depict what paints look like on a monitor. You could only have estimates that will be further thrown off by the users' color calibration settings.