Yellow line vs. Red line parking. - boxsterboy
My (overly) inquisitive 6-year old asked why some roads have yellow lines on them.

So I explained that double yellow lines means no parking, ever. And single yellow lines means no parking at the times shown on a sign.

As we were in London, he then asked why some roads have red lines on them.

So I explained that double red lines means no parking, ever. And single red lines means no parking at the times shown on a sign.

Of course, he then asked why the two different colours. Which reminded me of the very same question I asked when red routes were first introduced way back when. The Highway Code confirms the same legal status regardless of colour, and so I'm bound to conclude that red lines can only have been brought in after some very clever lobbying by road paint manufacturers. I wonder how many £millions of public money could have been saved by proper enforcement of the yellow lines, instead of introducing red lines!
Yellow line vs. Red line parking. - paulb {P}
I had always understood that red lines meant no stopping, none, not at all, not even for a second to let someone get in or out of the car - hence a stronger prohibition than yellows. Can't remember where this came from, though, and it is clearly not a view supported by the HC.
Yellow line vs. Red line parking. - Cliff Pope
This from userfocus.co.uk:

"Because London has more cars than it has parking spaces, driving in London is like a huge game of musical chairs. The lucky few have parking spaces by their home or office but the majority of us are doomed to drive around and around in circles hopelessly searching for empty stretches of kerb. A while back, this got even more difficult: on certain main roads yellow lines indicating a parking ban were replaced by red lines. Unlike yellow lines, single and double red lines ban all stopping, parking and loading."

Yellow line vs. Red line parking. - gmac
From the highway code:

Along the edge of the carriageway

Waiting restrictions

Waiting restrictions indicated by yellow lines apply to the carriageway, pavement and verge. You may stop to load or unload (unless there are also loading restrictions as described below) or while passengers board or alight. Double yellow lines mean no waiting at any time, unless there are signs that specifically indicate seasonal restrictions. The times at which the restrictions apply for other road markings are shown on nearby plates or on entry signs to controlled parking zones. If no days are shown on the signs, the restrictions are in force every day including Sundays and Bank Holidays. White bay markings and upright signs (see below) indicate where parking is allowed.

Red Route stopping controls

Red lines are used on some roads instead of yellow lines. In London the double and single red lines used on Red Routes indicate that stopping to park, load/unload or to board and alight from a vehicle (except for a licensed taxi or if you hold a Blue Badge) is prohibited. The red lines apply to the carriageway, pavement and verge. The times that the red line prohibitions apply are shown on nearby signs, but the double red line ALWAYS means no stopping at any time. On Red Routes you may stop to park, load/unload in specially marked boxes and adjacent signs specify the times and purposes and duration allowed. A box MARKED IN RED indicates that it may only be available for the purpose specified for part of the day (eg between busy peak periods). A box MARKED IN WHITE means that it is available throughout the day.

Red and single yellow lines CAN ONLY GIVE A GUIDE TO THE RESTRICTIONS AND CONTROLS IN FORCE AND SIGNS, NEARBY OR AT A ZONE ENTRY, MUST BE CONSULTED.

See www.highwaycode.gov.uk/signs09.htm for examples.

My understanding was that red was used to indicate you are on a major arterial route where stopping can cause major conjestion.
Yellow line vs. Red line parking. - ForumNeedsModerating
I believe (..but stand to be corrected by constitutional historians) that along with setting fire to a naval dockyard or attempting to subvert the tue monarch as head of the CofE, parking on red lines still (in statute) attracts the the death penalty. The last time in actual law was in 1767, when an unfortunate pie-seller, one Nathaniel Grimsdyke of Pudding Lane EC2, was convicted & hung at Tyburn. His last words were, reputedly, 'I only stopped to sell a pie! God bless King George!'
Yellow line vs. Red line parking. - paulb {P}
In response to and partial agreement with Woodbines, the last few things attracting (in theory) the death penalty were, to my knowledge,

1) high treason
2) arson in HM Dockyards
3) piracy on the high seas

As far as I'm aware 2) was done away with by one or other of the Criminal Damage Acts >35 years ago. The others are technically still there.

Parking on a red route, along with even looking at a bus lane from the driving seat of a car, apparently attracts a sentence of public flogging for the first offence, and death by hanging for second and subsequent offences - or would if TfL had their way, I'm sure :o)
Yellow line vs. Red line parking. - ForumNeedsModerating
..attracts a sentence of public flogging for the first offence, and death by hanging for second and subsequent offences..

Hear hear! Hanging once is too good for 'em I say!