2.1.1081 Part 1 Section 18.18.30, ST_FillId (Fill Id)

a.   The standard allows this type to be any unsignedInt.

In Office, the valid values for attributes that use this type are restricted by the count of fill elements in the fills collection ("[ISO/IEC-29500-1] §18.8.21; fills").

Excel reserves FIllId index values 0 and 1 for internal use. If these are overwritten in the styles.xml file, Excel will ignore the fill definitions and use its internally defined styles instead.