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if you have enough bandwidth there's no problem RECORDING colour. The problem is playing it back. The term "velocity error" is usually taken as meaning timing variation within a line. This will cause SC phase to vary along a line, causing hue errors, especially in NTSC. The high inertia of a helical drum means that velocity errors are small on helical VTRs. Unlike quadruplex where they were significant. Conversely helical has much larger timing variations on a longer timescale. Look at the hooking effects you can get on replay when the receiver's line sync can't follow the variations.
Colour under, as used on VHS, Betamax etc was a fix for lack of basic bandwidth. Some of the VR5000/7000 series Ampex and 1" IVC VTRs had sufficient BW to record colour directly and I think the colour replay add-on remodulated the chroma on to a stable subcarrier. Just as in a colour under machine but without changing the basic SC frequency.
SECAM could be recorded and replayed without problems as it's not sensitive to this sort of timing variation.
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