I was thinking red & black.
1) Not all trucks came with an air pump if was dirty then it wasn't used.
2) There may be a coolant switch that replaced the block drain plug. (plug is at the pan rail just forward of starter end plate). a yellow or light /green color would make sense
3) It doesn't seem long enough to go to a low coolant switch in the radiator tank.
Online kinda sucks for a schematic. You can take your thank you back if you want.