Authoryear style and how to make entries appear as Author:Year in the footnotes?

It should be enough to just redefine the delimiter format for nameyeardelim in the appropriate contexts (footcite and probably also smartcite)

documentclass[british]{article}
usepackage[T1]{fontenc}
usepackage{babel}
usepackage{csquotes}

usepackage[backend=biber, style=authoryear, autocite=footnote]{biblatex}

DeclareDelimFormat[smartcite,footcite]{nameyeardelim}{addcolon}

addbibresource{biblatex-examples.bib}

begin{document}
Lorem autocite{sigfridsson}
ipsum autocite{worman}
dolor footcite{nussbaum}
sit textcite{geer}

printbibliography
end{document}