Changing a RGB Color ingame


i want to have an Option to control the RGB Values ingame/with blueprint.
So for example i pick an object and get a Widget that let me control the Color of that Object by editing
the R G B Values in the Material Base Material

This is a good tutorial you can refer to:

And here is another one:

You most likely want to create Material Instance from your Base Material, which should have named Color property that you can access in blueprint. Create Material Instance and assign it to variable, set that material instance to your mesh and after that you can control all the material properties through BP, not just color.