SSS Color defines the brightness of the SSS. As a rule of thumb, I generally don’t set this to more than .5: even if I’m using a color, I tone down the color a lot.
Opacity defines the translucency of the material being scattered. A high opacity would result in very thin scattering, and not much blurring on the other side. Lower opacity means more scattering. As a rule of thumb, this should be lower than .7, otherwise the material does not scatter light much at all and it looks really weird. Even very thin paper will scatter light much more than what a .7 material would. If you have a material where light passes through it without scattering, you should be using a translucent blend mode, not the scattering mode. These are the only two options in UE4.
If you’re painting a SSS map on your character, you only need a single channel for Opacity. Thin areas like the ears, nose, and fingers will have a lower opacity than bony areas like the top of your hand. If your character has human skin, the SSS color would be red, as light passing through the fingers and ears would turn red from the blood on the inside. Think more in terms of value when painting the color map, and less in terms of texture, because SSS textures always get blurred almost beyond recognition. You can get away with only having a single channel for the opacity/value maps to define areas by how much light passes through, then multiplying the value map by a 3-vector to get the color you need. The only example of SSS maps that I have is on an embarrassing old test model, but it works.