2 Improved answer.
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Blitz 2D/3DBlitz 2D/3D, 154154 108 bytes

This produces exactly the same thing as the example given in the question (except for the anti-aliasing at the edges where colors meet).

Graphics 800,533
ColorClsColor 0255,35255,149255
Rect 0,0,267,533Cls
Color 2550,25535,255149
Rect 2670,0,266267,533
Color 237,41,57
Rect 533,0,267,533
SaveBuffer(FrontBuffer(),"a.png")

Output is saved in a.pngdisplayed and it looks like this:

enter image description here

Blitz 2D/3D, 154 bytes

This produces exactly the same thing as the example given in the question (except for the anti-aliasing at the edges where colors meet).

Graphics 800,533
Color 0,35,149
Rect 0,0,267,533
Color 255,255,255
Rect 267,0,266,533
Color 237,41,57
Rect 533,0,267,533
SaveBuffer(FrontBuffer(),"a.png")

Output is saved in a.png and looks like this:

enter image description here

Blitz 2D/3D, 154 108 bytes

This produces exactly the same thing as the example given in the question (except for the anti-aliasing at the edges where colors meet).

Graphics 800,533
ClsColor 255,255,255
Cls
Color 0,35,149
Rect 0,0,267,533
Color 237,41,57
Rect 533,0,267,533

Output is displayed and it looks like this:

enter image description here

1
source | link

Blitz 2D/3D, 154 bytes

This produces exactly the same thing as the example given in the question (except for the anti-aliasing at the edges where colors meet).

Graphics 800,533
Color 0,35,149
Rect 0,0,267,533
Color 255,255,255
Rect 267,0,266,533
Color 237,41,57
Rect 533,0,267,533
SaveBuffer(FrontBuffer(),"a.png")

Output is saved in a.png and looks like this:

enter image description here