Conifer bark mulches can be rich in tannic acid and chemicals called phenols which can have an adverse effect on growth in some plants making them less vigorous. Composted bark though finer doesn't usually have this problem.

I wasn't aware you could get coloured bark... it you can I'd be concerned that the dye in the bark could also affect the plants and the soil.

Yes, I wondered about that bit too???

Come to think of it I seem to remember seeing coloured bark at B & Q - yup
this place sells...
http://www.dandys.org/prod.asp?category=Coloured-Woodchip&CID=536

our local council used red and yellow on one of the roundabouts. bright but 6months down the line the red is a dull pink and the yellow looks like pine chippings.

Still question the toxins going into the soil along with the phenols

me too mg.