Exactly. ett rött äpple but en röd bok. And plural is röda: röda äpplen 'red apples'.
There are some exceptions like lila (purple) and svart (black) which have the same forms for 'en words' and 'ett words'.
The old loved children book "Sagan om det röda äpplet" - Jan Lööf And Youtube has it as a play: Search Junibacken och Sagan om det röda äpplet.
Easy Swedish :)
Yes, in fact rödt is an older spelling of rött that was abolished in 1906, but is still used in Danish and Norwegian. It was still pronounced as rött since you can’t have a voiced dental followed by a voiceless dental like that without the former assimilating.