Hello Kavya,
in case you select a PlatformPackage with RGB565 (e.g. NXP.iMX_RT_VGLite.RGB565), the framebuffer will have the color format RGB565. This means, all graphics operations into this framebuffer will have this destination color format.
In case you define a bitmap resource as native, it will be generated as RGBA8888 and provided as RGBA8888 image for the GPU. In case you define a bitmap resource as RGB565, it will be generated as RGB565 and provided as RGB565 image for the GPU.
I hope the topic is more clear now.
Best regards,
Manfred.