You want to keep drawing cards from the deck as long as possible. Drawing a 2 or 3 from the board is usually a waste of valuable time, unless you have multiple investment cards or a realistic chance of playing 8 cards.
Always play cards when there are no outstanding cards between that card and the most recent card of that color you have played.
Until the endgame, stall for time. The more cards you have a chance to pick up, the better your score will end up. Discard cards your opponent cannot use. Discards cards opponent might be able to use if you hold enough cards to know that they can't possible get more that 25 points. At start of game, i will discard singleton 2 and 3, and start playing 5. 4 I hold on and wait further developments. That is if the opponent has not started the color. If they have started, I might discard a 3 or 4 if they have played an uninvested 2, but would really avoid anything else they might use.
I never discard investment cards unless I have already started color and have enough cards that opponent cannot possibly get more than 25 total. Hold on to them util opponent plays a number card. Then you can dump them.
Endgame starts when there are 16 cards left in deck. Count how many cards in your hand you want to play. Keep stalling until the deck gets down to 2 times that number. Then keep drawing from the deck as you play cards until the number of cards in the deck equals the number of cards you want to play. Then you have to draw from the board to have time to play all of those cards. Really really good players may keep drawing from the deck anyway if there might be cards in the deck that are exceptionally valuable for either player.
When forced to make a play when you would rather stall, burn your weaker color cards first, giving you more time to grow the stronger colors. The big score comes when you can play 8 cards in one color. Won't happen every game, but when it does, it almost guarantees a win.
The toughest call is deciding when to start a new color with or without an investment card. General rule of thumb. Multiple the remaining number of cards in the deck by 2/5 and add that to the number of card points you hold in that color. If greater than 20, play the investment cards. If greater than 15, I would start the color anyway. You probably won't make it to 20 points, but you won't loose very much, and will by time to draw more cards to support your stronger colors. Standards change a bit if opponent has already started that color. I wouldn't play an investment card unless against opposition unless I was holding at least 17 points in my hand.
Do not be afraid to start a color with 16+ points even when there is little or no chance of getting more. Yes you will probably take a loss on that color, but if that stalls long enough to draw a high value card in another color, it will be worth it.
Always have fun!!!