While this thread can make for some very interesting discussions it must be realised that it will be complete conjecture and speculation! As far as I'm aware the camouflage of both the BDU's and the armour are simply there because "soldiers wear camouflage" there is no specific scripted reasoning beyond that.
The USCM pattern itself is basically a re-coloured variation of the US Vietnam era ERDL camouflage pattern if I recall correctly. Whether the colours were chosen for any real camouflage purposes or simply to look different to any currently in service camouflage is open to debate.
As for its effectiveness in the real world there really is only one way to find out. In your second post you mention effectivess in the Infra Red and Ultra Violet spectrums I suspect that since both the Screen Used and reproduction USCM BDU's have been produced to look "cool" they are unlikely to have been printed using the specialist (and thus expensive) IR reflective/absorbtive inks necessary to offer any genuine protection in these areas.
As for the contrasting camo patterns on the BDU's and armour probably the simplest and most realistic explanation would be the same as for the case highlighted by Kevin in Desert Storm and equally in Iraqi Freedom and not just among US troops British troops were also to be seen dressed in mixed camo patterns. This reason is quite simply economics, any force buys equipment best suited to it's most likely area of operations. BDU's being probably the cheapest component of a warriors kit can be stockpiled in varying camouflage schemes against possible need. But more expensive items like Armour, NBC Kit and Webbing are generally only available in the most likely needed pattern.
Of course if you want to stray outside the strict confines of the Aliens film then the Extended Universe is your oyster! The Tech Manual for example specifies;
USCM Technical Manual wrote:
BDUs are issued in several patterns for deployments to temperate, desert and jungle zones. The disruptive pattern of the fabric is printed in at least three colours, dependent on the design, and the different high-temperature dyes used in the print process are each designed to absorb infrared radiation at different frequencies, effectively breaking up the wearer's IR signature
The Tech Manual also depicts a very geometric and high contrast camouflage pattern in conjunction with one of the hostile environment suits but there is no reference to different patterned armours!

Then of course if we're straying into extended universe we can speculate on the thermal camouflage properties of the armour shielding the largest part of the human IR signature namely the Head and Torso. Or the use of James Bond style "adaptive camouflage" the basics of which I had figured out long before Die Another Day hit the screens and is not as fictional as some of 007's other gadgetry.
Well better close now hope I've given some more food for thought!

_________________
Cheers
Winch
"Yea though I walk through the valley of the shadow I shall fear no evil for thou art with me. And together we'll kick ANY bugs ASS!"
You know who you are!
