SDK: Parameter for LrColor with name and alpha value

#SDK - According to the API Reference, LrView static_text fields can have a "text_color" parameter that accepts a LrColor object to control the color of the text.

When looking at the LrColor definition, it can take various forms, including :

LrColor( name, a) ‌‌ Creates a color with the given name and alpha value.

So I tried:

text_color = LrColor("blue", 0.5) -- doesn't work

-- gives error "Invalid parameters for constructing an AgColor." when reloading the plugin

and I got an error saying : Invalid parameters for constructing an AgColor.

However, this works:

text_color = LrColor("blue") -- works

text_color = LrColor(.1, .2, .5) -- works

So what's wrong with the LrColor( name, a ) format? Am I using it incorrectly?

Thanks.