Here's an idea...give it good Dex and Cha progression, and give it a deflection bonus equal to its age category + Cha modifier. Assuming the yellow's Dex progression and the silver's Cha progression would yield a range as follows:
Wyrmling: 17 (+2 size, +2 Dex, +3 deflection), touch 17, flat-footed 15
Adult: 26 (–1 size, +6 Dex, +11 deflection), touch 26, flat-footed 20
Great Wyrm: 40 (–4 size, +12 Dex, +22 deflection), touch 40, flat-footed 28
Compare to the pink's breakdown:
Wyrmling: 14 (+2 size, +1 natural), touch 12, flat-footed 14
Adult: 26 (–1 size, +16 natural), touch 9, flat-footed 26
Great Wyrm: 41 (–4 size, +34 natural), touch 6, flat-footed 41
That looks pretty close, eh?