At the start of the game, examine the nobles on offer and see if there are several wanting similar colors. If so, focus on collecting those at first.

Cards vary a lot in price, even for the same color and victory point values. It's usually worth grabbing a cheap (3 cost) one if you can, especially at the start.

Make use of the cards you've bought. If you spent the time amassing lots of red and black, buy cards that cost red and black.