With the washed out effect, I noticed it too — suspiciously right after updating to High Sierra. Luckily, there's a fix. And it has to deal with macOS gamma values.
Sketch Gamma Correction Plugin
I don't know why Sketch doesn't bundle this into Sketch itself, but they released a Gamma Correction plugin to fix it.
Here's a description of the issue, and how the plugin fixes it:
We’ve identified an issue where bitmap images that are exported and re-imported into Sketch are appearing incorrectly and appearing washed out.
This is because macOS High Sierra renders certain gamma values in bitmaps differently to previous macOS versions.
If you’re using Sketch 47, we’ve fixed this bug already and all images exported from Sketch will have the correct gamma values and should render correctly. If you’re using Sketch 46 or earlier this bug will be fixed when you update.
However, if you’re creating and exporting bitmap images in other software and importing them into Sketch, you may still see issues with washed out colors.
To fix this, we’ve created a plugin that you can run on any Sketch documents that aren’t rendering properly. Simply click the link below to download the plugin, install it and run it once on each affected document. Please note, this plugin is only compatible with Sketch 47 or later.
And here's the page you can download it from.