Information from: BI Pride Flag 

From "The History of the BI Pride Flag"
by Michael Page

The first BI Pride Flag was unveiled on DEC 5, 1998. The intent and purpose of the flag is to maximize bisexual pride and visibility. This BI flag is for free public and commercial use.

As a result of volunteer work I was doing for BiNet USA, it occurred to me that if BI people were going to be visible at home, pride events and political rallies, we needed a BI Pride Flag!... read more

SYMBOLISM:

The pink color represents sexual attraction to the same sex only (gay and lesbian)

The blue represents sexual attraction to the opposite sex only (straight)

The resultant overlap color purple represents sexual attraction to both sexes (BI).

The key to understanding the symbolism in the BI Pride Flag is to know that the purple pixels of color blend unnoticeably into both the pink and blue, just as in the 'real world' where BI people blend unnoticeably into both the gay/lesbian and straight communities.