To convert a color image to black and white in Photoshop without using the grayscale function, you can use the Black & White adjustment layer. First, open your color image in Photoshop. Then, go to Layer > New Adjustment Layer > Black & White. This will create a new adjustment layer that allows you to adjust the light levels of the colors in your image. You can adjust the sliders for each color channel to fine-tune the conversion.
Alternatively, you can also use the Threshold adjustment layer to convert a color image to black and white. To do this, go to Image > Adjustments > Threshold. This will create a new adjustment layer that allows you to adjust the threshold of each color channel. By adjusting the threshold, you can effectively convert the image to black and white. You can also fine-tune the conversion by adjusting the sliders for each color channel.
In both cases, you can further fine-tune the conversion by adjusting the levels of the black and white channels. To do this, go to Layer > New Adjustment Layer > Levels. This will create a new adjustment layer that allows you to adjust the light and dark levels of the input and output channels. By adjusting the sliders, you can fine-tune the conversion to achieve the desired black and white effect.