GP,
Probably too late for you now but, rather than contrasting colours on ALL blades, a single contrast blade, as has already been mentioned (as per RAF SAR Sea Kings) works exceeding well due to the human eye's lower perceived rotor speed.
Problems can occur with the extra paint weight on a single blade causing rotor imbalance and manufacturing 'issues' (some reasons for comercial non-use). I've no doubt that your rotor speed will be a factor as well in relation to the eye's persistance of vision.
Small visual 'targets' such as microlights and Gyros ARE difficult to aquire visually due to various factors such as colour, size, relative speed etc, even from a low speed platform (such as another helicopter). A high speed FJ 'ups the odds' significantly. Worth considering avoiding obvious 'threat' areas and height bands if the choice available. Works for military crews as well - avoids us loosing no claims bonus!
Lookout and fly accurately bloggs.
RS15....and a bit