The Yellow after Red == "get ready to go, but, beware ?
Apparently the plan is to make mandatory this flashing-yellow arrow arrangement for left turns - in a four-section signal there'll be two yellow arrow indications, one will flash and the other won't.
Well, can you show us : Schematic, circuit setup and code. My test code is simple. It is just a Blink, but using multiple leds. 150 ohms is a bit high. Use 330 ohms or 470 ohms. Check the led polarity : cathode and anode. Check where the ground wire is, and the digital output pins are wired. My code is design to check the leds and connections... Checking the hardware basicaly.
Is 150 ohms to low?I calculated it with the formula: (Vsupply-Vled) / Iled, so (5v-2v) / 0.020A = 3v / 0.020A = 150ohms.