Whenever Paul and Timothy prayed for the Colossians they gave thanks to God for them. Note the many references to thanksgiving in this letter ( Colossians 1:3; Colossians 1:12; Colossians 2:7; Colossians 3:15-17; Colossians 4:2). [source][source][source]
"Paul could have meant that every time he prayed he remembered his various churches. Perhaps he maintained the Jewish practice of prayer three times a day (cf. Daniel 6:11; Acts 3:1; Didache 8:3), or perhaps he used the long hours of travel and of work in stitching to hold his churches before God (see also on Colossians 1:9 and Colossians 4:2)." [1][source]
Specifically Paul and Timothy rejoiced over the continuing demonstration of their trust in Christ as contrasted with their initial acceptance of Him as their Savior. This is clear from the Greek preposition en, translated "in." Furthermore the Colossians manifested self-sacrificing love for other Christians. [source][source][source]