You mean... making the line a certain color? In the trigger option, just change "Change colour and style to: (no change)". You can change it to any of the Custom colors.
^Some prickly ash bark (.*):([1]\d|2[0123456]|\d) gp
That would look for: a 1 followed by 'any' digit OR 2 followed by a 0, 1, 2, 3, 4, 5 or 6 OR any single digit by itself. So, anything from 0 to 26, but not higher. Just make sure to click the check box next to 'Regular Expression'. You may also have to adjust for correct spacing, like if the line reall appears as:
Some prickly ash bark blah : 23 gp
then you will need to add in the spaces before and after the ':' to fix it.
I tested my version and it works correctly. However, as I mentioned, you never gave an actual line showing what arrived and "gasp!" you have a space after the : in yours that mine didn't account for. Add that space, so it is:
^Some prickly ash bark (.*): ([1]\d|2[0123456]|\d) gp
and it should work. Though, from what yours shows, you may need to add .* to the start. You only need () around something if it is a) a sub string and/or b) you want to return it in a wild card, so you don't need the () around any .* parts you don't actually need for something.