Apple removed the ability to select the graphics setting for the built-in 10.5x Remote Desktop application…. But, this command:
defaults write com.apple.ScreenSharing controlObserveQuality n
…changes the quality of the connection’s display, based on the value of n:
1 = black and white
2 = grayscale
3 = 8-bit color
4 = 16-bit color
5 = full color
Which can be used as a workaround for that slider. Sadly, needing this work around doesn’t appear to be changing anytime soon. Snow Leopard does not restore this ability, and this workaround is still needed.