I bred my baby guppies about 6 months ago. There now living together (3 of them) in a suitable healthy fish tank and yet have still not gained the colour on there tails.

Is there something wrong? or does it take time?

It depends on many different things.
A healthy tank-You say you have a healthy tank.
Temperature of the water-78-82 degrees.
What you are feeding them-You need to feed them guppy fry food. It can help them grow faster.

They will normally gain color around 3 months but some get it later. Don't worry they will get their color.
Females normally aren 't as colorful, but they still have color. So it will not be that.

Are the new guppies female? they might not get any color then. My guppy babies took a real long time , over 6 months I'm sure, for the males tails to develop. Good luck!