It's difficult for me to propose changes since I use my favourite colours in my mod obviously. So if you ask me for a certain shade of blue for the US, I will probably favour my own pick. Why else would I have chosen it?
@Korea
I always loved the combination yellow and brown (both pastell). Hence I use a strong yellow for China and brown for Korea. With the Vicky-engine light and pastell colours look awful, so I had to go dark brown for Korea.
Any contrastful colour will do for Korea. Blue, green, brown, purple...
@Colombia and Venezuela
I would only change Venezuela. Simply make it lighter. IIRC I took Austria's white for Colombia and added a stint of yellow, so it will have this vanilla-touch (vanilla as in the colour, not the game). I would pick that colour for your Venezuela. It doesn't change the overall colour scheme, but only enhances the contrast.
You can simply download my mod and take the RGB code of Colombia if you like my idea.
@USA and shoreline
Yepp, a closer look at your US-shore and I know what you mean. And trust me, I know how much different the colours look in-game than the actual RGB code is supposed to look like.
This also makes RGB-proposals difficult as I don't know how the colour will actually look in-game.
This blue ( 0 61 245 ) looks decent for the US (at least in theory).