One approach...
Place the background clip on track 1 and the counter on track 2. If desired, add the POSITION effect on track 2 to reposition the counter.
Program the length of the counter clip to match the length of the background - 15 seconds in this example.
Specify a mid-green color, choose the font, size, etc. Other colors will affect what hue choices are available in the next step...
Apply the HUE effect to the counter track. Slowly move the slider to get the first color. Click the small green + sign above the graph to create a keyframe.
Move the cursor to a later position and slowly move the hue slider to another color. This will create a second keyframe...and so on.
The horizontal distance between keyframes determines the speed of the transitions. More space = slower; less space = faster.
As it was programmed in this example.
You may not get precisely the colors you want, but there will be a change - like this...
Record_2022_10_17_18_13_57_775.mp4