I don’t have any specific info regarding assembly point signs, but I did have a quick glance of the British Standards on safety signs (BS5499 parts 1 to 6). The relevent regulation is The Health & Safety (Safety Signs and Signals) Regulations 1996.
The green is pantone coated 3405 (85% cyan, 65% yellow).
The text and symbol are white.
The border should be white and at least 0.025 of the sign height.
The text should be Helvetica Medium.
I don’t think the signs have to legally comply with the above, but it is certainly good practice.
Ivan