The colour preference is up to you when going for a second budgie.
I personally like to pair blue series with green series budgies.
If you want another female, by looking at the colour on the cere you will be able to have an idea on gender. Young females will have light bluish mingled with white ceres, in time the white colour will spread more evenly on the cere.
When first coming in to breeding condition the colour on the cere will gradually tan till reaching a dark brown colour.
Young males will have uniform pinkish/purplish coloured ceres (no hint of white) and in most colour mutations the cere will turn to a royal blue colour upon reaching maturity.
RIP sweet Tito (Summer 2008 - January 17th 2013).
You are missed and never will be forgotten.