I have a toroidal transformer which has two 115V primaries and two secondaries rated at 25VA each (2x18V each capable of delivering 1.389A). I've attached the datasheet here. I'm a bit confused about using this with 230VAC primary voltage. Can I connect the two primaries in series (blue+green connected, 230V mains AC between Brown and Violet) to get 230VAC and the two secondaries in parallel (Red+Yellow and Black+Orange) to get 18V at 2.778A? Color codes are specific to this datasheet.

 

Color codes are specific to this datasheet.

I assume you mean the colours on your transformer leads are the same as the datasheet. In which case your assumptions are correct. split primaries and secondaries are normal.

 

The attraction with Toroids, they are easy to add an overwind if needed, either a separate winding or add to, or subtract from an existing secondary.
You have to keep the original Va rating in mind when doing this.
Max.