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Masses of Cowslips, and Primroses with Bluebells will look pretty. That will give you a show of yellow and blue, but if you want more colour, Primulas are available in many different colours in early Spring.

Some tulips flower in April. Its a bit late to find them now but you might be lucky and there is just about time to plant them. Do check the flowering time though, its usually printed on the packet. Bluebells are a lovely idea but usually appear in early May where we are - such a lot depends on the weather at the time. Check out hyacinth too. Any of these will look great among the primroses and polyanthus.

you may be able to find plants in flower in the local garden centre at the time and you can plunge the pot into the borders for instant effect. This will allow you a wider range of plants and apart from the initial cost you can select what you like.
in addition to what's been suggested, obviously daffs especially some of the mini ones. Some of iris reticulata may also be in flower.
but as already said the weather will also be crucial. we could be under snow!
don't discount shrubs and their foliage, eg, hebe, leucothoe etc.

If the borders are around a lawn with a marquee on it, then that is a lot of border. It is going to need a huge number of plants so will need a lot of money throwing at it at this late date.
The plant supply need to be organised asap and I would suggest trying to find a local garden designer to help. They all have contacts in the plant supply area and will know be able to get plants at wholesale prices.