Are you trying to change the graded colour range settings?
If you open "Settings > Colour ranges..." you can define how a graded colour scheme is mapped onto a range of values.

Vijay - if you want continuous shades above and below a threshold value, you can set these using:

Data > Manage fields > {field} > Options > Value colouring

If you want an 'alert' type display with values over a threshold value say, red, and values under it green, you need to add a 'bucketing' formula to categorise the continuous range of values as either above or below threshold, and then set the corresponding category colours using:

Data > Manage fields > {field} > Options > Value order, colour and shapes

See also the discussion here:

http://forums.visokio.com/discussion/comment/5101/#Comment_5101