Hello, i did what you did, empty the rad from the bottom hose on the rad, leave it disconnected. Then get a water feed from a hose pipe or watering can and put water in through the expansion tank until the water rushing coming out of the hose at the bottom of the rad runs clear.
With regards to antifreeze, alot of us use Paraflu available from Shop4Parts.
Also I got this info from the alfa145.co.uk forum written by the admin which you will find very useful.
The system takes 8.3 litres of coolant. I suggest you do a 50/50 mix of coolant/water. Ideally you should use distilled water as opposed to normal tap water as it contains no nast impurities that could affect the cooling properties. You should be able to get distilled water from any motor factors.
Regarding the coolant, DO NOT mix the new pink antifreeze with the green antifreeze already in your system. Green and blue coloured antifreeze is glycol based. Red and pink coloured antifreeze is organic acid based. If you mix the two different types they will react and potentially cause damage to your engine. You can happily mix blue and green as they're both glycol, or red and pink as they're both organic. If you can, I suggest you take the pink antifreeze back and get some green or blue instead. If you can't take it back then the alternative is to drain all the old green stuff out, fill with clean fresh water and run the engine to dilute the remaining green antifreeze then drain it out. Repeat this flushing procedure a few times to ensure that you have completely cleaned the system of any glycol antifreeze then it will be safe to use your pink organic antifreeze in the system.
ps. Thanks to GialloEvo for writing this