There were so many Imperial regiments raised in the 30 Years War that to distinguish them they would change the various colors upon which the imperial eagle was displayed - the small shield that often is shown on the breast of the imperial eagle was itself striped red, white, red (as is the modern Austrian flag), so it looks like that first regiment has adapted those colors for their regimental flag (there were many variations; e.g. one unit quartered their colors red and yellow, with the eagle overlaying that; another used grand quarters, with red and white horizontal stripes in the first and third quarters, and the yellow field with black double-headed eagle in the second and fourth).
All that said, I guess I'm stumped as to whether the tabards should follow regimental flags or those of the national flag. The regimental colonel paid for his trumpeteer's clothing and equipment (which would include the trumpet banner and tabard, if worn), so I guess it would be up to the Colonel. There aren't many examples still extant. I have never seen a picture of an Imperial tabard of the period that wasn't yellow, but certainly doesn't mean there weren't any. This is sufficiently abstruse that we may never get a definitive answer.