B1 veneers seem yellow. What can I do about it? (Photo)

Unfortunately, I agree with you.  The shading does not match the youthful skin tone and youthful look of your face.  The only way to rectify this is by removing the veneers and having them redone.  As far as the dimensions of the front incisors, I think they fit in well and fall within normal proportion for your smile.

B1 is a little warm by default, but I also believe 8 upper veneers suit you better.  Your premolars are pushed in qmd thus look hidden. 
Get a 1M1, thats a beautiful,  natural and medium white shade.
Bleach shades are 0m1,om2 amd 0m3

In my practice, I will always try in the veneers and let the patient view them in various light sources (operatory, waiting room, outside daylight...) before cementing them in permanently. This way I get the patients feedback and can still alter the color with various colors of cement. However, since your veneers are already bonded to the teeth with the permanent cement, they can no longer be altered in color.

Unfortunately the only way for you to brighten your new veneers is to cut the ones you have on and order new brighter/whiter ones. Be warned, if you want to go whiter, it is not advised that you only get 6. If you only have 6 that are whiter, then the side of of your smile will appear darker like a dark hallway. We call this the “buccal corridor” phenomenon. Your dentist did only 6, but matched to your natural teeth color. If you go white, it is advised that you cover your entire smile with 10 or 12 veneers.

Hello,If you want to have the veneers that make harmony with your face and I know that when you suck them, you should choose an expert.If you are not happy with your veneers, you should remove them and put them on, this should be done with your dentist or visit another one.You can have all 20 teeth, have a top and ten button. In South America the procedures are at a good price and with highly certified and highly experienced professionals

From the photos you submitted- It looks as if the shade is very coordinated with you other teeth. if you went to any of the bleached shades, there is a chance that those 6 front teeth will 'stand out' and look less natural. 
Having said that- were the veneers tried in and shown to you prior to binding them?? Generally if they were and you approved it- You dentist may be hesitant to change them. You should certainly make an appointment to see them to discuss it though. 
In my opinion, if you really want them whiter, then you should think about doing a total of 8 -10 upper teeth for uniformity,