http://www.twcenter.net/forums/showthread.php?607479-Celtic-units/page2
It's highly probable that Celts wore leather armour. The Linothorax (a modern term coined by archaeologists) was popularised by Peter Connolly, but because of the expensive and time consuming effort needed to make armour out of linen, it is highly likely that Greek (or Celtic) armours of this type were made of hardened leather as you say. Also these armours tended to be painted in many colours, and white was popular with the Greeks, so the Celtic types might have been similar in colour too.
Here's a richly decorated modern reconstruction of a Celtic 'Linothorax' (or Greek style cuirass) :