Vol.1 Ch.6, Ch.17: Dash (1st couple found in ch1-3, extra in 6) Vol.2 Ch.18: Dash (2nd couple found in ch.4-6) Ch.19: No Color (all chaps)
Vol2 ch31-32 is already included in Doushiyoumo Nai Keredo
Vol2 ch36-37: Doushiyoumo Nai Keredo (2nd couple found in vol2 ch7-9.2)
Vol2 ch38-39: Doushiyoumo Nai Keredo (1st couple found in vol1ch1 to vol2ch6.1, vol2 ch9.3-10.1)
In short, you should have read first Dash and No Color to understand vol1, and Doushiyoumo Nai Keredo for vol.2 (author makes crossovers with other works though)
Vol.1 Ch.6, Ch.17: Dash (1st couple found in ch1-3, extra in 6)
Vol.2 Ch.18: Dash (2nd couple found in ch.4-6)
Ch.19: No Color (all chaps)
Vol2 ch31-32 is already included in Doushiyoumo Nai Keredo
Vol2 ch36-37: Doushiyoumo Nai Keredo (2nd couple found in vol2 ch7-9.2)
Vol2 ch38-39: Doushiyoumo Nai Keredo (1st couple found in vol1ch1 to vol2ch6.1, vol2 ch9.3-10.1)
In short, you should have read first Dash and No Color to understand vol1, and Doushiyoumo Nai Keredo for vol.2 (author makes crossovers with other works though)