This is from a 2005 draft standard on "National Rollingstock Lighting and Visibility Standards":
10.1 - Locomotives should have livery applied that has areas of high visibility colour applied to the ends and sides of the vehicle.
10.2 - A forwards facing area of high visibility colour shall be applied the leading ends of locomotives.
10.3 - The colour of high visibility areas should be one of yellow, orange, orange-red or red within the CIE chromacity co-ordinates of Table 1 [can supply if needed]
10.4 - Table 1
10.5 - The colour of the high visibility areas should have a minimum luminance factor, when new, as detailed in Table 2.3 of AS/NSZ 1906.4
10.6 - Suitable AS 2700 high visibility colours include:
10.6(a) - Yellow Y11 to Y15, Y22, Y23, Y26
10.6(b) - Orange X11, X12, X13, X24
10.6(c) - Red R11, R12, R13, R22, R23
10.7 - The forward facing area of colour shall be not less than 1 square meter in area with a minimum continuous height or width of 0.6m
10.8 - The forward facing area of colour may be divided by the features of the front of the rollingstock if necessary.
10.9 - Each section of a divided colour surface shall have an uninterrupted area of 0.4 square metres with a minimum continuous height or width of 0.6m
10.10 - If the front of the rollingstock slopes from vertical or is rounded, the area of coloured surface and its vertical and/or horizontal dimensions shall be increased in order to produce the equuivalent required areas and dimensions when projected onto a vertical plane.
10.11 - The high visibility colour areas shall be cleaned and maintained to prevent obscuring or fading below half their new luminance factor.
Whether or not this is changed since then is unknown, but we can observe the main point, 10.3. This is the reason for yellow and red on the front of 7007 and presumably other refurbed Sprinters.
It was pointed out by mjja on VICSIG (where this was originally posted, I should note
) that the yellow may not meet the requirements of 10.7. Whether or not that's changed, or if the red (i.e. all of the colour patches) are counted together, I couldn't tell you.
It's also notable that N473 with the white stripe doesn't actually comply with these regulations, at least not in this draft form, as white isn't covered in 10.3, and I doubt the stripe is 1m^2 in size as per 10.7. I also doubt that N-class red is of sufficient luminance to comply either.
The odd thing is that find find N473 to be more visible front-on than 7007 - I think it has something to do with the contrast of the reflective white on red, rather than just a huge bit of yellow.