Although you can’t make it automatically pop up when you switch to that layer – you can save custom colors to your Swatches tab.
First, make sure the Swatches tab is open (under Window)
Set the custom color you want as the foreground color
Float the cursor over the blank bottom area in the Swatches tab area
When you get the paintbucket, click, give your color a meaningful name and click OK
You can even save and load whole swatch setups – which is great when you’re working on a client website with a very specific color palate.
A computer once beat me at chess, but it was no match for me at kick boxing.
–Emo Philips