Hey!
What you’re seeing here is the result of the standard UE tonemapping taking place. This also happens in the preview window of the material editor. If you switch either window’s view mode from ‘Lit’ to ‘Unlit’ you will see your colours reproduced exactly as you expect them.
Here are a couple of relevant forums posts:
-
[ Washed out Colors from Tone Mapping: How bad it really is, and how to fix it](Washed out Colors from Tone Mapping: How bad it really is, and how to fix it)
-
[ Dissable Color Grading and Tone mapper completly or…!?](Dissable Color Grading and Tone mapper completly or...!? - #14 by Gildor)
At the end of that second one you can see a really simply post processing material setup for replacing the tonemapper, so that it will look like the unlit mode.