UHF antennas

The 4.5 db aerial will give greater reception in open countryside and if fact will probably be all you need.

The 6 db has a different radiation pattern more suited to "hilly" areas, but of course if the transmitting and receiving vehicles are both in "valleys", reception may be poor anyway.

Colour and thickness have no bearing on the quality of the aerial at all.

Probably more important is where the aeial is mounted and whether it is an independent ground plain type. This will have the greatest impact on performance.

Strac, it would help if you gave the name / model numbers of the antennas.
By the gain figures given, the 4.5 dB antenna would be 2 x 1/2 wave elements fed in phase and the 6 dB antenna would be 2 x 5/8 wave elements fed in phase. These type of antennas are collinear antennas.
As both are of the same height, it sounds like the 4.5 dB antenna could be a ground independent type where as the 6 dB one requires a ground plane. With this in mind, the 4.5 dB antenna should perform to spec where ever it is mounted (within reason). It is also probably has an elevated feed point. This is good!
The 6 dB antenna performance will be compromised if the ground plane is not suitable. This antenna sounds like a Mobile One "Mega Blaster"
Antenna colour should not affect performance, just as long as the material is UV stabilised.

A note on antenna gain. In this case it is assumed that the decibel gain figures are using a standard 1/4 wave as a reference. Usually, gain is termed as dB gain over something else. A 3 dB gain is the equivalent of doubling the transmitted output power. Gain is achieved by concentrating the transmitted radiated power, the higher the gain, the greater the concentration.

Thanks fellas !
Both are mobil one, the white one is col45m (yes colinear) with about a 30cm metal section that go'es into the spring bullbar mount. The other is m476 mobil one with only a fitting at the end to screw on to the mount, It's impractical to roof mount so I'll mount on the bullbar or possibly the guard. As far as terrain go's it's pretty mixed but hilly more often!