Just to further clarify what's been said above: U space is U space, whatever the color of the U. When you see that U without a slash through it, an instant upgrade is available. As an MVPG, you can access that instant upgrade space two ways: via a GGU on any fare class except G, R, or T; or "free" on an H fare or above.

What the various instant upgrade search options and the color-coded U's are doing is showing you the lowest available fare in a qualifying fare class for the type of instant upgrade you want to use. So the choice you're making when you want an instant upgrade and there's U space available is whether to buy a cheaper fare (anything but G, R, or T) and burn a GGU, or buy a more expensive fare (H or above) and upgrade for "free." If you do neither, you're still eligible for an upgrade once you get to the MVPG window, regardless of fare class, but your odds are worse (often a lot worse).

Which leads me to a comment on your Hawaii plans: your post isn't completely clear as to whether you're planning two solo Hawaii trips or one trip with a companion, but if it's the latter, you might consider using the AS credit card companion pass and booking an H fare. You'll pay more for the first ticket (usually around $700-800 RT between HNL and SEA), but the second ticket is the same $99 + taxes as it would be in the cheapest fare class. Both tickets will book into H and will upgrade at purchase if there's U space available (using the MVPG companion upgrade benefit), so you end up with two seats in first for under $1000 RT, and you save your GGUs for solo trips.