The pink coolant was used in all new toyota/lexus 2004 or later model year cars, and is good for 5 years at least, and 10 for the first fill. In short, if you have an IS250, it will be intended to have pink.
The lexus service manual does not recommend flushing for normal maintenance, because flushing will dilute the pre-diluted coolant. If you use a concentrated coolant, then it doesn't matter if you flush and leave water behind - you add the correct amount of concentrate, then top up with water.
The pink coolant is a vivid "hot pink" colour. There is no aftermarket coolant with a similar colour - they are all red, red-orange or deep purple. If you shine a torch on the coolant expansion tank, you will see the colour clearly.
In general flushing is only needed if you have a corrosion problem, and the cooling system is full of crud. The pink stuff is really good at preventing corrosion, so I would be rather surprised if a 2006 model had a genuine need for flushing.
The lexus recommended maintenance is draining the coolant and refilling without a flush. Ideally, you should drain the whole radiator and engine. The radiator drain is easy. The 2 engine drains are difficult to find. (One is behind the alternator, the other is behind the dipstick)
If the coolant has been changed, then you may as well just flush it - you can then, either flush with pink, then drain and refill with pink (very expensive, as it is nearly £10 per liter for pink). Or flush out with water and use a generic coolant that is similar to toyota/lexus specifications and just change it regularly (perhaps every 3 years instead of every 5).
Annoyingly, there is no generic or 3rd party coolant on the UK market that has similar specifications to toyota pink. Cheaper options are the use of toyota red coolant, or a generic OAT coolant (a "G30" formulation is probably better), but note that the reason toyota switched to a their formulation was that their red was designed for 5 year changes, but didn't work quite as well as they had hoped.