If this bit rate suggestion is the cause of your problem . . no way to tell without some experimenting.
Firstly, you haven't stated just what your shadow resolution settings are. Make sure they are enabled for the project. And the easiest way to check the impact on your clip is to change them down to a lower resolution, in steps, to see if or when the colours go away. A point about shadow files is they are provided to make editing smoother to do, not show the final hi-res result. So a shadow file for a 4k clip can be any resolution that's good for your PC, but that is really dependant on the power of your cpu (fast clock and lots of RAM helps). However the default shadow res of 720x480 should be good enough for any clip when it's only used in edit. The larger the clip size (4K?) then longer it will take to create the shadow file. Please note that the 720x480 size is an NTSC number, which despite my location being in a PAL country is the default/lowest number on display in properties. If it were correctly adjusted for PAL, that resolution would be 720x576 which is just not included in the list. But 720x480 is workable, since the shadow is only used for edit and the rendered result is your edit
actions applied to your original 4k clip.
In another editor, a project can be specified to have particular properties: so as a start, change the properties there. Even if you load a clip with different properties, it's processed at the project specified rate. Later, in rendering, quality can be reduced by reducing the bit rate specified in the render, to enable the result to fit onto a disk.
But we aren't there, so I spent a bit of time reviewing the comparisons/options for PD18.
1. During edit.
I note that you said ' .. insert a shadow files to the timeline . . ' so presumably it is occurring during edit, and then only in particular clips. Clearly the 4K shadow for those files showing colour at the bottom of the preview window still have a high bit rate even at the shadow resolution.
So one option is to pre-process the clips to reduce the bit rate using a program like Handbrake - most often used to stabilise a variable frame rate, but there are facilites to change the bit rate as well.
This method changes the actual source file, so you might not want to take this approach. In which case, use the lowest shadow file resolution available and if the colours are still there then live with it during the edit phase (see footnote).
2. During Render/produce
During produce there are options for each target media type (eg MPEG-2 or H264 aka mp4) in a list box to choose the bit rate (Mbps) you want in the result. Pick one you think suits - experiment - and render it. Play the result to see if that colour bar is still there on the clips in question.
Footnote.
My prior experience with this effect was with a specific type of camera. And most camcorder users use the default settings in the device, which are the manufacturers version of "optimal". However, the output settings of the camera could also be adjusted in terms of format, frame rate and resolution (bit rate per frame). So adjusting this setting in your camera is also worth investigating.