I guess the Melnorme bridge turns purple because it is a side effect of a highly efficient broadcast-jammer turned on when the transaction begins, in order to avoid badly intentioned people (like the Druuge maybe) to listen about the information and deals the Trade Master is conducting.

- Dthunk

I agree with it being merely the color for setting themselves into a trading mood and mode.

- Phwendo-004

Best theory I've heard is that purple is a pleasant, soothing color, helping with trading negotiations subconsciously. Of course, since you don't barter with us Melnorme chappies...

- Greenish_9

I think the Melnorme Bridge turns purple because the Melnorme's use color coding. The colors represent their feelings. Purple is for trading I guess. Did you notice that their bridge turns red when they are threatened?

- JR

The "purple background" trick is kept for the rich-and-very-curious races. When they pay the huge amount of credits for the info, the Melnorme tell that the whole thing is only about getting money for it. Certainly, their high morale wouldn't allow the Melnorme to fool anyone, so the background trick is worth the money: the buyers can also use it to become rich. And besides, they just wanted to know, and they got the answer.

- Krilin

I had always thought that had to do with their color/power changing gun.

- Tzzakz

I think that the Melnorme changes it's bridge into violet maybe because it wants to get us into deeper and special business. If we had enough credits to pay for the it's cost, it would be much income to them.

- Changerds

I think it is because purple is a peaceful color and puts you in a relaxed mood for trading.

- LindaG

Well what i think is just that the Melnorme are selling crap (or at least it isn't worth 120.000 credits) Ah well they are just like the Druuge! -rosy sphere (useless) Perhaps they bought the purple bridge from the Druuge and when they figured out it was useless crap, they still wanted to make a profit out of it and sell it to helpless starfaring races (Spathi are exactly the ones who would buy it). And it's weird that they'll answer the BLUE bridge question free of charge (IT IS INAPPROPRIATE AND SOME OTHER WORD I CAN'T THINK OF RIGHT NOW) . And you'll have to pay for the purple bridge.
My guess is also that it is that they are
-ready to trade
OR
-friendly mode
since Blue = Aggressive mode and Red = Neutral mode.