[GilB57]

to get the default aircraft letters the same as the squadron letters on your skins

Not so easy because letter's color vary depending on user's install (for example I use Mat Manager which have different colors... maybe also Planeshine Mod ?)
The default game letters have a different shininess than skin.... so the color may appear different depending on point of view :roll:

You may use a paint program to suit the colors for your install ?