Is it permissible for my colleague to sell me the car that he bought from me in installments, whether for cash or in installments, after six months?

It is permissible for the questioner to buy the car from his colleague because there is no collusion or prior agreement between them to return the sold item. His colleague has owned it for six months, and this indicates that the second sale was not a condition of the first sale. Ibn Hajar and Al-San'ani pointed out the scholarly consensus on the validity of the second sale if it occurs after a period, unless there was a prior condition for it.