This is details of app top, as it appears on device, bottom as it appears in designer. I've checked many times, all four reds are set to same red. I've tried setting the red to many different shades of red, none come even close to being red. Fonts are all set to default, the difference looks worse on device than the picture shows.
Slider color setting.
Any explanations or fixes?
Can you share your blocks and extensions used
This is due to the construction of the slider. Nothing you can do about it.
Since you are obviously very interested in layout/design, I don't understand why you are using the "Classic" theme. The Slider then looks really old-fashioned and ugly.
For example, it could look like this:
Well, I'm old fashioned and ugly. 
----- Old anyway. I really don't like the little ball and thin line of the other views. If I could change that ball to some sort of image that would be cool. At some point I'll change the play, stop buttons to some image. Something of my own design.
Sometimes I try to make things easier to see for old tired eyes. I really dislike the dark themes the designers are using lately, hard for me to read.
I didn't use images, I used Google's material icons (MaterialIcons-Regular2.ttf)
If you want more control over your slider colors, use a thin Canvas with a fat line:
https://groups.google.com/g/mitappinventortest/c/eA_T9NWTa6A/m/SZTS48_FAAAJ
