To unlock the alternate colors for the planes there is an ace placed in each mission that you must find and shoot down. Here are the planes and locations for each mission.
1. F-5 just north of the allied airbase
2. A-10 north of the enemy airbase
3. F-16 South of the second radar station(the westernmost one)
4. Mirage 2000 Northwest of the westernmost radar jamming plane
5. MIG-29 South of the far western pumping station on the map
6. F-14 North of the northenmost group of enemies on the map
7. TND-IDS North of the solar tower
8. F-18 after refueling during the mission the ace will appear to the east of the main combat area
9. F-15C North of the center beach combat area
10. RF-01 Far upper northwest corner of the map, northwest of the sub base
11. SU-35 The ace will appear with about 90 seconds left during this mission
12. F-2 North of the stonhenge gun base
13. F-15E After clearing the radar balloons, enemy fighters will appear, the ace is in the north east
14. F-22 Ace is in the north east of the map
15. EF-2000 Ace will be in the center of the map when the mission starts(between the two southern combat zones)
16. F-117 Ace is located northwest of the westernmost combat zone(Due to stealth sometimes hard to find, head 2 grids north and 1 west)
17. F-15A Ace is located in the far northwestern corner of the map, past the city
18. S-37 The ace is located north of megalith.
The F-4 is available after completeing hard mode, the X-2 is available after completeing expert mode.
The aces are sometimes difficult to spot on radar as they don't show up until you get close. When you do find them they will be red on the radar instead of the usual enemy fighter colors of yellow or orange.