Hi. Without the model number of the combo and the TV there is things i can't check but there is some things i can tell you to verify.
Most modern TV now have a component input (the 5 connection cable, red green blue and the 2 audio cable). That is the connection that give the best quality. Then some TV and combo can be connected with SVideo connection, that is the second best connection for quality. Those two are usually DVD exclusive on a combo. Lastly there i a composite output on the combo (yellow and the two audio cable) and that one is for the VCR and not exclusive for it as the DVD part can also output to it (with a slighly less quality signal).
From what it look to me is that you used one of the two? (i am not sure of all the available connection you have on it) DVD only connection. Ideally you would use the component connection for DVD and run a second wire from the composite outout. If you don't mind the lower quality and prefer less wire clutter you can use both DVD and VCR part with a single composite cable. If you use two cable, you need to select the right input with the TV remote as they will be connected on different input on the TV.
If you use that red white and yellow cable and don't have black and white picture, it mean that you indeed have it into the right composite output on the combo. In such a case you should have both DVD and VCR on the same TV input. Did you adjust the tracking on the VCR so that it does send clear signal?
I have to go to bed soon, if you look on the back of the combo and you can get me the model number i would be able to look into things in the morning and come back to you. I can't really say how the tracking is working without knowing much about the unit (often there is an auto tracking but doing it manually may be requited with some tapes).
Humm, something might be wrong with it. Those device often have very distinct circuitry for the DVD and VCR part. If you still have the manual, the model number should be on top. I may also be able to know the model by cross referencing the remote model number.