If you need it to be longer just copy and paste the clip at the end of the first clip when you bring it to the timeline. If you need it even longer than that then copy and paste it again at the end of that clip.
So then why not take that solid color to the timeline, copy and paste and edit it till you get the length that you want. Then export that video and when that’s done bring that whole solid color clip to your project so that you can do your HTML Overlay on it.
Alternatively you could create a color image in a graphics program such as Gimp or Ms paint.
Export as a JPEG or PNG. Drop that into Shotcut.
The default duration of an image is 4 hours. Trim it to the desired length in the source viewer and drop it on the timeline.
Actually, this just occurred to me. Take the solid color video file from Shotcut, keep it in the preview window (or put it in the timeline), go to File menu and pick “Export frame”. That’ll take a snapshot of wherever the playhead is at in the video as a png file. Save it to wherever you want it on your computer. Then take that png file and put it back to the Shotcut playlist. On the playlist, pick the png file and go to Properties. There you can set the duration you want for the image and it’ll let you put a duration that’s much more than 10 minutes. After that, bring it to the timeline.