Before we even get to Beltane or May Day we must first cross the great threshhold of Walpurgis Night, the last day of April, sometimes known as May Day Eve. This is one of the most demonised festivals of the entire year, a time when demons, devils, fairies and just about anything else was about and waiting to attack. Or was it?
The fact that something has been so thoroughly demonised usually suggests that certain aspects of the festival resisted all attempts to Christianise it. So maybe the celebrations and activities of this night are a faint remembrance of a once-important Pagan celebration.
One of the main features of Walpurgis night (which falls tomorrow, so I'm telling you all this in advance in case you want to have a go at it) is bringing plants into the home in order to prevent evil from entering. This may have originally been done in order to bless the home or indeed to protect it, and ‘May’ can refer to any flower in bloom at the time. This could include, for example, the Marsh Marigold and not just the May or Hawthorn flowers. Hawthorn’s male counterpart, the Blackthorn, was said to be guarded by fairies on May Day Eve, and traiditonally people seem to have avoided picking it if they could.
Plants brought indoors were hung over doorways, windows and even letter boxes, (all places of entrance or exit) or scattered outside the front door, especially on Walpurgis night or May Eve. Primroses were particularly popular for this purpose, although I think they're far too pretty to scatter!
Protective plants were also placed wherever animals were kept, such as barns and stables. Branches of Rowan were sometimes brought indoors, but these had to be broken off and not cut with a blade. Other popular protective plants were Primroses, Elder leaves, Kingcups, Marsh Marigolds Rushes and Flags. This preference for yellow blooms and green branches for Beltane may be a way of honouring sun Gods, including the Celtic Bel.
Other names for Beltane were the Feast of the Bright Fire, Garland Day and May Day.
Usksider
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...and I always assumed the preference for yellow blooms was simply because they were what was available at the time... Bel never entered my head.

Thanks Tylluan, for yet another interesting and informative post.