Host redirection not working for second host in iRule

You may try Daniel_Wolf  solution but your iRule has clear mistakes as the second "if" statement is in the first "if" statemet.

when HTTP_REQUEST {

if {[HTTP::host] equals "test.com" and [HTTP::uri] equals "/"} {
HTTP::redirect "https://test.com/redlab/login.htmll" }
if {[HTTP::host] equals "lab.com" and [HTTP::uri] equals "/"} {
HTTP::redirect "https://lab.com/bluelab/login.html"}
}

or even better

when HTTP_REQUEST {

if {[HTTP::host] equals "test.com" and [HTTP::uri] equals "/"} {
HTTP::redirect "https://test.com/redlab/login.htmll"
}elseif {[HTTP::host] equals "lab.com" and [HTTP::uri] equals "/"} {
HTTP::redirect "https://lab.com/bluelab/login.html"}
}

Alse this free article for writting irules (the "then" statement is just optional):

https://community.f5.com/t5/technical-articles/irules-101-02-if-and-expressions/ta-p/283431