Blue Serial Number

Mmm...interesting question. So perhaps...before 2001 (when they started printing the year of issue on the note) they needed to change the colour because of the serial number repeats?
I think in later years when they exhausted the serial run in any given year (two alpha then 6 numeric) they added a suffix of the letter A.

And seasonal greetings to you, have a good holiday.

182 20 Piso VG VF UNC
ND (1997); 1998-. Orange and blue on multicolor underprint.
President M. Quezon at left center, arms at right. Cental Bank Seal
Type 5. Back: Malakanyang Palace. Watermark: President M.
Quezon.
a. Signature 14. Red serial #. ND. FV FV 2.50
b. Signature 14. Black serial #. FV FV 2.50
c. Signature 15. Black serial #. 1998; 1999. FV FV 2.50
d. Signature 15. Blue serial #. 1999. FV FV 6.50
e. Signature 16. Blue serial #. 2000. FV FV 5.00
f. Signature 16. Blue serial #. 2001. FV FV 6.00
g. Signature 17. Blue serial #. 2001. FV FV 4.00
h. Signature 17. Black serial #. 2001; 2002; 2003; 2004, 2005. FV FV 3.00
i. Signature 18. Black serial #. 2005; 2006; 2007; 2008. FV FV 3.50
s1. As a. Specimen. â€" â€" 50.00
s2. Uncut sheet of 4. Specimen. â€" â€" 100.