"The butterfly is partly blue and partly yellow."
Translation:Tá an féileacán idir a bheith gorm agus buí.
'between blue and yellow' implies a shade of green - does the 'a bheith' give the meaning of 'partly' then?
That's too literal a translation of "idir" - idir ghorm agus bhuí actually means "both blue and yellow", not "between blue and yellow" - the a bheith just implies that it is being blue and yellow at the same time, so partly blue and partly yellow.
Here's a few snippets from a search for idir a bheith - the meaning changes with the context.
Tá cumasc anois ann idir a bheith ag ithe sa bhaile agus ag ithe amuigh - " There is a merging of eating at home and eating out"
fear a roinníonn a chuid ama idir a bheith ina mhúinteoir tacaíochta agus ina fhoilsitheoir leabhar agus CD-ROM trí Ghaeilge "A man who divides his time between being a support teacher and a publisher of books and CD-ROMs in Irish"
an difríocht idir a bheith fostaithe agus féinfhostaithe - "the difference between being employed and self-employed"