Special case OpenGFX as a "base" set to allow recolouring signals.

src/gfxinit.cpp

@@ -42,6 +42,7 @@ static const SpriteID * const _landscape_spriteindexes[] = {
 
 /** file index of first user-added GRF file */
 int _first_user_grf_file_index;
+int _opengfx_grf_file_index;
 
 /**
  * Load an old fashioned GRF file.
@@ -228,6 +229,16 @@ static void LoadSpriteTables()
 	LoadNewGRF(SPR_NEWGRFS_BASE, i);
 
 	_first_user_grf_file_index = i + 1;
+	_opengfx_grf_file_index = -1;
+	uint index = i;
+	for (GRFConfig *c = master; c != NULL; c = c->next, index++) {
+		if (c->status == GCS_DISABLED || c->status == GCS_NOT_FOUND || HasBit(c->flags, GCF_INIT_ONLY)) continue;
+		if (c->ident.grfid == BSWAP32(0xFF4F4701)) {
+			/* Detect OpenGFX GRF ID */
+			_opengfx_grf_file_index = index;
+			break;
+		}
+	}
 
 	/* Free and remove the top element. */
 	delete master;

src/rail_cmd.cpp

@@ -1871,8 +1871,11 @@ static void DrawSingleSignal(TileIndex tile, const RailtypeInfo *rti, Track trac
 		sprite = (type == SIGTYPE_NORMAL && variant == SIG_ELECTRIC) ? SPR_ORIGINAL_SIGNALS_BASE : SPR_SIGNALS_BASE - 16;
 		sprite += type * 16 + variant * 64 + image * 2 + condition + (type > SIGTYPE_LAST_NOPBS ? 64 : 0);
 	}
+
+	uint origin_slot = GetOriginFileSlot(sprite);
 	extern uint _first_user_grf_file_index;
-	if (!is_custom_sprite) is_custom_sprite = (GetOriginFileSlot(sprite) >= _first_user_grf_file_index);
+	extern uint _opengfx_grf_file_index;
+	if (!is_custom_sprite) is_custom_sprite = origin_slot != _opengfx_grf_file_index && (origin_slot >= _first_user_grf_file_index);
 
 	if (!is_custom_sprite && variant == SIG_ELECTRIC && IsRestrictedSignal(tile) && GetExistingTraceRestrictProgram(tile, track) != NULL) {
 		if (type == SIGTYPE_PBS || type == SIGTYPE_PBS_ONEWAY) {