I used PSE 4 on my Mac for a while, until 6 came out this year. I never noticed any reduced colour range support - it did 16-bit RGB, though I have no trouble working in 8 bit, which gives millions of colours (256 levels for each of the three colours). 256 colours (as in GIF files) is pretty basic and would give a really grainy effect in almost any photo or render.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)