Most famously, the household troops of the House of York itself wore parti-coloured livery jackets in 'murrey and blue' - murrey being a kind of claret colour - with the gold 'sunne in splendour' motif of the House of York embroidered as a livery badge on the left breast (usually shown as the murrey side).
But of course the Yorkist armies would have included the forces of many different lords and nobles at different times in different phases of the wars, and many of these would have clothed their retinues in their own livery colours and badges. Then there were many more troops who who mercenaries or paid men who probably came with their own clothes and equipment, and simply wore a livery badge or field sign pinned to their coat to show their alliegance to one or other faction...
So there was no Yorkist 'uniform' as such for the entire army...
But half-in-half coats in murrey and blue would be a safe bet to start with
(The blue is halfway between royal and navy)
Highly recommend the paintings of Graham Turner who illustrated all the Osprey WOTR books for inspiration...
You can see his prints here:
http://www.studio88.co.uk/acatalog/medieval_prints.htmlI have several, including this one hanging on my wall