To clarify for OP, the colours he's talking about are on the HP bar and not the colour of the shield itself.
I'm guessing your physical damage shield is meant to be a Talon/Zed type of counter.
Does it build out of Seeker's Armguard?
If no, what items should build into it? Is it a general HP + AR build-up or is it an AD/AP + AR build-up?
If yes, does the upgrade give more AP + AR? Seeker's already gives 45 AP, 40 AR when fully stacked. What shield value is it; a flat amount like Hex/Maw or a % max/missing HP?
Are you going to make the Seeker's passive unique to prevent stacking this item with Zhonya's or just allow mage armour stacking?
How much is it going to cost? Mid or late game gold cost? This is important because too cheap and you basically get an early Seraph's Embrace; too expensive and you're better off getting Zhonya's.
What are you trying to solve with this item? Burst AD? Sustained AD damage?
Who is this for? Everyone? AD vs AD? AP vs AD?