Some modders opt for one compound only in their mods. I have always given people the choice of at least 3 compounds, but others may do it differently.
Basically, it depends on the mod.
If the tire files support more than one compound, you'll have to look into the rfm file, where the freesettings variable can be set:
// 1 = steering lock
// 2 = brake pressure
// 4 = starting fuel
// 8 = fuel strategy
// 16 = tire compound
// 32 = brake bias
// 64 = front wing
// 128 = engine settings (rev limit, boost, and engine braking)
// For example, the default free settings are: steering lock (1) + brake pressure (2) + fuel strategy (8) = 11.
// Possible values are -1 (use default value) up to 255 (which equals 1+2+4+8+16+32+64+128).
FreeSettings = -1