Ideally, you would want to save this as a new colour in your colour table: it would then appear as an available colour, as you request. I don't see that there is any way to do this.
Here are two ideas:
o Put the cursor in your coloured text.
o Click the Format Paintbrush icon in the Standard toolbar.
o Drag the paintbrush over the new text. To avoid painting the paragraph formatting as well, hold down Ctrl whilst you release the mouse after the drag.
o Put the cursor in your coloured text.
o Go to Format | Styles and Formatting or click the Styles & Formatting button in the Formatting toolbar or press F11.
o Click Character Styles.
o Click New Style from Selection | New Style from Selection.
o Give your new character style a name.
You now have a character style with the captured colour, which you can apply to other text in the normal way.
I trust this helps.
Brian Barker